I'm having a similar problem - iPod 4th gen, iPhones 4 & 4s, new iPad Mini, & iMac '08. The wifi has been very spotty the past 3 days. Sometimes there's a signal, most times there isn't. It's worked okay for over a year prior to this.
I read in these forums to put the modem in bridge mode and connect a router (which I don't have yet). Do I need to be selective with what router I buy, given all the Apple products?