Summary
Veil is generally considered a marine biased map. Long straight corridors and easily defendable siege points combine to make it easy for marines to capture and hold positions. Also, due to the very spread out nature of the res nodes, it is fairly easy for marines to hold a number of res nodes easily.
Key points on the map

Red
Emergency C-12 Nozzle
A popular relocate for marines, allows easy domination of the right-hand side of the map, and is within walking distance of 2 hive siege locations to boot.
System Waypointing
Seals off the most direct route to Sub hive, and allows fast access to both Double res, and Cargo hive.
East Junction
Seals off the most direct route from Pipe hive to Double res, and from Pipeline hive to Marine start.
Allows fast access to Double res, Pipeline hive and Cargo hive siege points.
Green
Sub Hive
This hive is fairly open, fairly easy to spawn camp, however it's extremely hard to siege, due to all siege points being so close to the hive. Your best bet to take this down would be a jetpack rush.
Cargo Hive
This hive is fairly easy to siege, it has 2 easily defendable siege points, although it isn't particularly easy to jetpack rush it due to the low and uneven ceiling.
Pipeline Hive
This hive is very easy to siege, it is very open and has a high, even roof - making it ideal for a jetpack rush or spawn camping.
However, the C12 siege point is ridiculously easy to defend.
Yellow
These are the parts of the map your marines should be defending in the early seconds of the game.
They should be holding these points until around 0:45, when most of your base should be up (IP, armory).
Siege Points
All siege positions below have been combat tested for accuracy and ease of defense. The sieges in the picture below are placed as far away from the hive as possible while still reaching.
Pipeline Hive


Siege Point 1
This is just around the corner from C12, and is fairly accessible. There are really only 2 entrances that will cause you hassle, the 2 large doorways. As long as your marines concentrate on covering the 2 doorways this is usually the easiest siege location to defend on the map. The sieges should be placed hugging the wall as close as possible to give the defending marines more room to move about. You can also (until b6 (probably)) CC block the hive-side doorway and/or vent, making it *that* much easier to defend.

Siege Point 2
This siege point can be very difficult to defend if there is mroe than one fade harassing your marines. A key point to defending this area is to have a fair amount of mines in the two corridors leading to the hive, as these will seriously hinder the progress of any skulks attempting to attack, and will deal severe damage to fades as there is little room to blink over them.
Keeping the sieges hugged up tight to the wall in the corner will give your marines more than adequate room to defend.
Note: A Grenade Launcher can be put to very good use due to the relatively enclosed space of the corridors leading to the hive.
Cargo Hive


Siege Point 1
This siege point is ridiculously easy to defend. One long corridor, the only easy access to the hive.
Placing the sieges like shown, is the furthest back that they will hti the hive, but you may want to move the TFac forward, and place the sieges along the side to give the defend marines a better LoF (Line of Fire) and more room to defend.

Siege Point 2
This siege point can be very hard to defend due to the closeness to the hive itself, and the amount of large entry points where fades can harass your marines undeterred. Shotguns would be the best investment for defending this siege point.
Placing the sieges tight to the wall will give your marines more breathing room and less to get stuck on.
Sub Hive


Siege Point 1
This is a nightmare hive to siege, mainly due to the siege spots being so close to the actual hive.
Mines are a must for this area. Keep the turrets at the end of the corridor and your marines should be able to hold the location fairly easily.
Common Map-Related Strats
C12 Relocate
This is a very popular relocate, as it locks down one of the 2 main map control points (C12 and System Waypointing). It is within walking distance of 2 hive siege points (Cargo and Pipe) and has good access to nearby res nodes at double/topo/pipe.
With this relocate marines will dominate the pipeline/cargo side of the map.
System Waypointing Relocate
This is another popular relocate, as it locks down one of the main map control points (C12 and System Waypointing). It is within walking distance of 2 hive siege points (Sub and Cargo) and has good access to nearby res nodes at double/sub/overlook.
Jetpack Rush
This strat is particularly effective on ns_veil due to the large open hive rooms, which will give your jetpackers lots of room for manouvre, and unless the hive rooms are OC spammed, you will see those hives go down quickly.
C12/System Waypointing Lockdown
This is rarely used strat, and involves locking down the two major map control points (C12 and System Waypointing) with either a relocate and a PG at the other, or 2 PG's.
This can be difficult to set up due to your marines having to push for a PG near the aliens start hive, no matter which one they start at; however the rewards are more than worth the risk.
With PG's at both these locations your marines will effectively have map dominance by easy access to every early alien res node, and their start hive.
I would suggest placing a TFac at the PG nearest the alien start hive, but mines will also suffice if you have enough marines pouring out of it.
It also gives easy access to most siege points.
Credits
An "intrepid | " member (Can't remember which one) - For "constructive criticism" (ffs n00b com sieges won't reach from there) of my Cargo siege point.
[HD] Clan Members, for continued "constructive criticism" of my commanding.
The "Bry's NS Server" Community, for helpful hints and tips.
-=THV=- Clan Members, for not ejecting me when things go wrong.
keep it *G* angsta, for the tip on JP rushes
(If I've missed anyone please contact me and I'll add you to the list)
Picture Links
http://img31.exs.cx/img31/8623/overview2.jpg
http://img46.exs.cx/img46/6964/overviewcargo.jpg
http://img16.exs.cx/img16/9084/overviewpipe.jpg
http://img44.exs.cx/img44/4979/overviewsub.jpg
http://img86.exs.cx/img86/2774/veil_guide_c12base.jpg
http://img86.exs.cx/img86/634/veil_guide_cargosiege2.jpg
http://img44.exs.cx/img44/9210/veil_guide_cargosiege1.jpg
http://img44.exs.cx/img44/2041/veil_guide_EJfarm.jpg
http://img86.exs.cx/img86/3016/veil_guide_pipesiege1.jpg
http://img86.exs.cx/img86/8589/veil_guide_pipesiege2.jpg
http://img44.exs.cx/img44/340/veil_guide_subsiege.jpg
http://img9.exs.cx/img9/6813/veil_guide_SWfarm.jpg