I am having a similar issue but mine will connect to my wifi at home and the 20 minutes later it will quit working, when it does that I look at the properties of the router and both the router number and the DNS numbers have changed, the last group went from .254 to .251. t starting to work a couple of days ago but this morning it has gone back to the same problem I just discribed. And to make matters worse when I get to work I try to get onto the wifi at work and a notice comes up telling my that my password is wrong.
I bought my 4s the day it came out and had no issue up until about a week ago, I have spent many hours on the phone with Apple senior tech support and AT&T techs and did everything that asked me to do with no results, I can hook up to a public wifi with no issue which the Apple tech is a 100% indicator it is a router issue which do not not believe and he said he had heard nothing similar regarding my issue so I doubt their well on their way to fixing the problem.