Preview at WWDC likely in June, followed by September launch.
WWDC 2012 Already Scheduled for June 10th-15th?
Tentative WWDC Dates:That document did accurately reflect the 2011 conference dates, but was not correct about the 2010 dates, emphasizing the tentative nature of the information. But adding some fuel to the speculation is the Moscone Center's event calendar, which currently shows a generic "Corporate Meeting" listed running from Sunday, June 10th through Friday, June 15th next year.
2012: June 10th - June 15th
2013: June 9th - June 14th
2014: June 1st - June 6th
2015: June 7th - June 12th
Such generic listings "Corporate Event" and "Corporate Meeting" frequently appear as placeholders for WWDC well ahead of time, although they are of course not exclusive to Apple and are used whenever booking entities do not wish to publicly disclose their identities.
Even if the "Corporate Meeting" listing does refer to WWDC, Apple is certainly capable of adjusting its schedule, although busy conference schedules can make it difficult to do so as things begin to book up. Consequently, it seems reasonable to speculate that Apple and the Moscone Center have at least penciled in the June 11th-15th dates for next year's WWDC, but we'll certainly be keeping our eyes on the schedules to see if anything changes, as they have on occasion in the past.