Our best guess is that it’ll be sometime this fall -- they might choose to announce it at the yearly iPod event in September, for instance, which would mean that it’d be available in time for the holidays. The iPhone 4 was announced at Apple’s Worldwide Developers Conference last June, though, so the iPhone 5 might debut at this year’s WWDC ... or it might not appear until next year. Who knows?
It’s possible that Apple’s next big iPhone announcement will be for a 4G Verizon iPhone, instead of the iPhone 5. Networks like Verizon are rolling out super-high speed, 4G wireless infrastructure, and competitors’ phones are already taking advantage of it. Don’t expect Apple to lag behind them for long!