Preview at WWDC likely in June, followed by September launch.
Apple Prepping for Notebook Refresh?
Meanwhile, Apple has notified Best Buy that they would have limited MacBook shipments over the next two weeks. While retail inventories alone aren't typically an accurate predictor of Apple product refreshes, combined with these other reports it may be more suggestive.
Apple has been rumored to be working on new aluminum MacBook Pro designs that will adopt features of the current iMac and MacBook Air. Intel's recent Montevina introduction also provides Apple with suitable processor upgrades to power new laptops.