If you iPhone or iPod is not able to connect to your WLAN after the update to iOS 5, you should first check the settings of you WLAN-Router. If you do not broadcast your SSID change this setting.
Apple recommends to disable the hidden network:
http://support.apple.com/kb/HT4199
http://support.apple.com/kb/HT4199?viewlocale=de_DE