Most game engines aren't designed to handle the polycounts of your typical Poser figure. The latest Quake and Unreal engines can handle about 3000-5000 polygons per player model, but for a fighting game you'd only have two figures onscreen at a time, so you might be able to push it a bit further without serious loss of performance.

The upcoming Doom 3 and Half-Life 2 engines may support higher polycounts; I haven't heard exact numbers yet. The footage I've seen of HL2, however, looks very promising.

Still, even the standard P4 figures have over 16000 polys apiece, and that's not counting clothing and hair. It'll probably be a few more years before the real-time software and hardware is ready to handle that level of detail comfortably.

You'd have to completely re-bone the meshes, since Poser's skeletal system doesn't translate into any existing game's skeletal animation features.

And if you plan to distribute the game in any way, there are all those nasty legal issues ....