I have the same problem, heres a pretty simple solution. Go to your local radio shack, and buy 3 Y shaped splitters for mini stereo plugs. Plug them into the 3 sound card slots for center/sub, front, back channels. Plug your speakers and headphones into the Y splitters, and use the inline volume on your headphones to control their volume, and the volume knob for your speakers to control theirs, you can turn off the speakers while still using the headphones this way, and you degrade the signal VERY little. I searched for a control box for some time before I figured out that it was almost totally unnecessary if I just used the volume on the speakers/headphones to turn them on and off and hook them both up at all times.
Not much data on it yet, but you can assume Fallout 3 will have something of this sort when it is released. Hellgate London is also close, although it is all instanced. Both are more futuristically set.