Preview at WWDC likely in June, followed by September launch.
Tentative Upcoming WWDC Dates Leaked?
A convention calendar (PDF) hosted on a Japan-focused site for the San Francisco Convention and Visitors Bureau reveals scheduled dates for Apple's Worldwide Developers Conference (WWDC) for 2010 through 2015. While the list carries a "last update" stamp of May 2009 and lists incorrect dates for Macworld 2011, listings for many other future conventions do appear to be accurate and thus the listings for WWDC may represent tentative schedules for future years.
Tentative WWDC Dates:
2010: June 27th - July 2nd
2011: June 5th - June 10th
2012: June 10th - June 15th
2013: June 9th - June 14th
2014: June 1st - June 6th
2015: June 7th - June 12th
The 2010 dates match closely to speculation based on a "Corporate Event" listing for Moscone West in the June 28th - July 2nd timeframe. While that entry has since been removed from the Moscone Center's calendar, similar entries have accurately represented WWDC dates in previous years.
WWDC 2009 was held from June 8th - 12th, and Apple announced the schedule in late March, suggesting that we may soon see an official announcement of dates for the 2010 conference.