I have the same problem. I bought my iMac in March, worked fine till june. Than the problems began. Apple replaced the Airportcard, but 6 weeks later i have the same problem. I've tried a few things like deleting my Bootcamp, Downgrade to 10.6. and than to 10.6.3, deleting and recreating new network and so on but nothing helps. It is mad, sometimes it works normal for 10-20 minutes, but than it stops. Reboot doesn't matter. Tomorrow i have an appointment at the apple store in Frankfurt. Not happy about carrying my 27" to the store unless they don't do their job as well (Fingerprints behind the glass at picking up, than they clean the glass, but then i had more fingerprints outside). If i would work in the same way in our garage with customer cars, my boss would beat me up.
Btw: Why apple doesn't have an pick-up service for the desktop modells?
I will give them three tries, if they can't solve the problem i'll change to an other modell...but no iMac.