Having same problem myself. I have a BT Home Hub 3 which the following connect to iPhone4 (2), iMAc, Macbook, Macbook Air, Apple TV, PC, Wii,iPad 3 and Kindle. Network seems to work with everything else. Rebooted router several times, changed channels several times. Hooks up briefly sometimes after changing channel. Switched phone off and on as well with hard reset. Only thing haven't done is completely wiped phone and restored to factory defaults.
Any ideas? Will have to change for another brand of phone if it doesn't resolve itself in the next few days.