Preview at WWDC likely in June, followed by September launch.
iPhone 5 Production Underway for an Expected Fall Launch
For reference, Mac Otakara also cited similar sources when they reported that the iPad 3 first went into production in January 2012. The lead time between production and launch for the iPad 3 was approximately two months with an official launch date of March 16th, 2012. That could suggest that we could see the new iPhone as early as September, though the production timelines for the iPad and iPhone may differ significantly.
Meanwhile, the Wall Street Journal had previously reported that the iPhone 5 would begin production in June, and all signs have pointed to a September or October launch for the new iPhone.
Leaked iPhone 5 parts have suggested Apple will be adopting a two-tone backing with a central metal panel. Mac Otakara reports that in the production models the back panel have a naked aluminum surface and a partially glass covered portion. They also note that a small hole that was seen between the lens and flash in the prototype parts is now absent.