UPDATE: The new iPad (3rd generation) will ship March 16, 2012.
The iPad with Retina display was announced on March 7, 2012 and became available for pre-order on the same date.
Various reports point to an A6 processor in development that will not be complete until early 2012. Assuming the iPad 3 uses Apple's next generation processor, this would put the release date in March 2012 or later. This would fit the current schedule of iPad hardware releases to date:
Original iPad preorders: March 12, 2010
Wi-Fi release date: April 3, 2010
iPad Wi-Fi + 3G release: April 30, 2010
iPad 2 announcement: March 2, 2011
iPad 2 release date: March 11, 2011
Some rumors have pointed to an iPad Pro or iPad Plus model coming to shelves in fall 2011. This top-end iPad could have a higher resolution screen or other features to set it apart from the iPad 2, while not being a complete redesign.
Stay tuned for more information as it becomes available!
UPDATE: New rumors say Apple could release the iPad 3 on Steve Jobs' birthday, which would put the launch date on February 24, 2012.