I've been thinking about the same thing lately, mainly because I have computer speakers that support Dolby Digital/ProLogic II, which my stereo does not. Therefore, I thought it would be nice to hook my 'Cube up to the computer and play through there, but it turns out that there is a small lag between DV cameras and BTV Pro/QuickTime/other capture app, so the games are unplayable. I've been investigating other ways, but what I really wish is that someone would make a consumer-grade PCIe video capture card. I'm not interested in spending thousands for a Kona 3 here.
On the other hand, if you have the cash, go for an Elgato EyeTV EZ, which is advertised as being good for playing games through your computer. I might check out the Hybrid mentioned earlier, though. It appears to be cheaper