I have the same issues with my iMac (it's the previous "generation" with a radeon 5xxx) and a Windows PC using the wireless internet connection at the same time. The Windows PC is fine, most of the time, while the mac just disconnects again and again. The fun fact is, when the mac runs Windows XP (via bootcamp), it still happens to the mac. I assume they have either some poor wireless device built in and/or some driver issues.
Then again I'm on a damn old G router, it might be that it can't handle to many devices connected at the same time via wireless at once. My connection is 32.000 kbit/s down 2.000 up, for real (if I plug in the cable, not via wireless obviously). That should be way enough, too
Anyway, if you have a new router, it's unlikely to be the connection itself, since I assume it can handle several devices better than old routers do.
Which leaves me with blaming the wireless device in macs
