Preview at WWDC likely in June, followed by September launch.
iPhone 5 Pre-orders Set to Begin at 12:01 AM Pacific Time Tomorrow [Update: Confirmed]
Apple sales representatives have been telling customers asking about availability that pre-orders will begin at that time, and both Verizon and Sprint have announced 12:01 AM Pacific launches for their own pre-orders. AT&T has not yet officially released a pre-order starting time.
Last year, Apple was nearly 45 minutes late in starting pre-orders and experienced significant issues with online processing for quite some time after they did launch, and many users had better luck pre-ordering through their carriers.
Update 10:24 AM: Apple has confirmed to Wired that it will begin taking iPhone 5 pre-orders at 12:01 AM Pacific Time tomorrow.