I too have had this problem with both iPhone and iPad dropping my wifi network and now neither are able to join the network again. This has only occurred since the last iOs update. had problems with them dropping the network in the middle of something, but was usually able to reconnect after powerdown and reboot, now neither will reconnect.
Apple support says it's my router and modem, or my password is wrong. Nope, not those as my wii, desktop and roomie's laptop all connect just fine. The devices see the network, but are unable to join.
I am thinking this is a software/iOS issue.