Yes the iOS 5.1 is having issues with the WiFi staying connected as well as the WiFi reconnecting to known networks. I just upgraded my iPhone 4 from 5.0.1 to 5.1 today and so far it seems to be a lot nicer than iOS 5. There are many new apps that are actually fully functional right now, but I cant use these new apps unless I have WiFi Connection. SO what I did was, I re-checked everything that I installed on the phone as far as the apps and iOS 5.1, to ensure that I had done it correctly. After this I knew that this wasn't my error, because I installed totally fresh and wiped everything today before updating. I went into the General settings and tried to reconnect to WiFi, which it did so, then turned the phone to sleep and woke it up. NO WiFi connection!!!! SO, just go back to settings, General, then to Reset. Once here, after pressing Reset, it will ask to " Reset all settings, Erase All Content and Settings, Reset Network, Reset Keyboard, Reset Home Screen Layout, and lastly Reset Location Warnings". Choose "RESET NETWORK SETTINGS". This will rest the network settings in the phone and turn off to a White Apple and Reboot. Once back in the phone, folllow the same procedures as connecting to a Network and select the one that is yours. Connect and put the phone to sleep. Once you come back up, there should be confirmation that WiFi is connected. So good luck.
