It's not an NS-specific problem, it happens to all goldsrc games. If the window loses focus, by accidentally moving the cursor over onto my other monitor and clicking whilst in commandermode, and you tab back in, the screen goes black and stays black. You can still hear the game sounds and move a cursor around, hearing the "ming ming" as you mouseover buttons, but you can't see anything.
This has only started happening as of the introduction of these new steam community features. I figured this was as good a place as any to notify people of this. I'll mosey on over to the steam forums and see if there's any threads on the subject over there.
For the record:
AMD 3700+
ATI X1800XT 512 meg
AC'97 audio.
Latest drivers for mobo, video and sound.
--Scythe--