Another Philly-area (Delaware County) user here experiencing the same problem. I've been in touch with Apple all day today and, long story short, the official company line is that their servers are overloaded from launching on three carriers, everyone downloading iOS 5, etc. I spoke to one person today who had me do a network settings rest, a full settings reset, and then, when neither of those worked, backup and restore the phone through iTunes. This also did not solve the problem, and when I spoke to another person about the issue after completing these steps, he told me that these steps wouldn't have solved the problem because it is a server issue.
I eventually got on the phone with a supervisor and explained to him what was going on. I am able to get weather when I ask specifically for Philadelphia, but when I dont specify a city (and let it rely on my location) or ask for my location by name or specific towns around me, I get the same error everyone else has been having. The supervisor put me on hold and, using an iPhone 4S he had, tried asking for these cities by name and had the same problem. He made note of the issue and told me he would let the engineering team know what the issue was. He seemed genuinely interested in helping me out, but I can't say for sure whether this will bring about a solution.
Long story short, this problem is definitely on Apple's end. I think it's strange that locations do and do not work within the same general area, and I can't think of a reason why this would be the case. I don't know about anyone else, but other than the weather, Siri works fine for other location-based questions. I guess we will just have to use another method for getting the weather until Apple works this out.
