Preview at WWDC likely in June, followed by September launch.
Apple Retail Stores Begin Prepping for iPad Launch as First Third-Party Retailers Open
Apple's retail stores will be opening at 8:00 AM local time to begin selling the new iPad, but some third-party retail partners will be opening even earlier. In fact, our friends at iFixit have traveled to Australia in order to be among the first to obtain an iPad for teardown purposes and were first in line at a Telstra retail store in Melbourne that opened just a few minutes ago at midnight. The carrier is even celebrating the event with a meter square iPad cake.
Update: iFixit now has its iPad in hand and will begin tearing it down shortly.