Pre-orders and previews begin April 10, launches April 24.
WWDC 2010 Likely from June 28-July 2nd
The schedule lists a "Corporate Event" occupying Moscone West during those dates. For the past two years this "Corporate Event" placeholder has accurately predicted the dates for WWDC (2008, 2009). Apple tends not to announce the official dates of WWDC until well into the new year.
WWDC has become of increasing importance to Apple as it now represents the only regularly scheduled launch event for the company. In the past, Macworld San Francisco anchored the January date as a second regular launch venue, but Apple will no longer be attending Macworld. Apple has launched a new iPhone model during the past two WWDC keynotes, and Phil Schiller even defined the iPhone's annual product cycle as a June revision.