Can I "Forget this Network" when the wifi network is not in range?
Seems like no way to do this at the moment. Workaround:


  The only solution seems to be to rename a spare access point to the SSID of the network you want to forget, connect to it with the iPhone, and choose the option to forget the network. This kludge seems like a lot of work and something that an average user probably doesn’t know how or want to do


via http://www.novainfosecportal.com/2010/07/08/forget-this-network-pretty-please/