I had the same problem with the previous gen 20". I got it 2/14 and constantly had the problem. Although I got it Feb this year, it was built early 2008. Don't know if that means anything. Anyway, when I took it back the first time, they saw that it was the problem and they replaced the wifi card and the logic board. I took it home and the problem seemed worse. Except this time, when I took it back to the Apple store they were never able to recreate the problem. So they ended up letting me exchange it without a restocking fee for the new iMac. I got the 24" and paid the difference. It's been almost a month and not a single problem.
I have an old Linksys G router by the way. My windows laptop and my macbook never had any problem at all.