yes to the first question. I can ping and visit the server on the kid across the hall's laptop using both his wired and wireless IPs (which are different). However, I believe win2k only uses one of them to actually do stuff... he gets 10 or 11 megabits per second peak, not 21.
I know the beowulf project has some (*nix) driver for shotgunning two regular NICs - maybe you could do something similar with a wireless and a wired.
and for the 2nd question, thats how the whole wireless network at the university works... they run cat5 to the access points.