I had this problem all the time with iOS 4.x and my home access points (which were totally up to date). iOS 5.1 was a godsend in this regard (though I waited for it to be jailbroken).
I don't have this issue too badly with a new iPhone 5 / iOS 6.0.1 -- I DO notice that the wifi bars/strength go up and down though, where before they were rock solid strong signals (I have two WAPs, one at each end of the house).
What worked for me previously was to use our wireless G only access point (ugh). I have also read that the work around is to use WEP instead (double ugh, I do NOT recommend that).
Perhaps 6.0.2 will get wireless right --
♬ Music is life ♬