Preview at WWDC likely in June, followed by September launch.
Lion Promo Materials Begin Appearing in Third-Party Stores as Apple Plans Overnights
As noted by Italian site Macitynet.it [Google translation], promotional materials for OS X Lion have begun appearing in third-party retail stores, with a location of retail store chain Euronics displaying a large free-standing banner as well as smaller table-top signage highlighting the new multi-touch gesture support found in OS X Lion.
The new signage advertises OS X Lion as being "now on all new Macs" and encourages users to "take a tour" of the new operating system. The machines on display have not, however, been updated to Lion yet.
Meanwhile, other signage promoting the sale of standard App Store gift cards to cover the purchase of OS X Lion has also popped up in at least one store of Dutch retail chain Dixons. Gift cards not only allow customers to effectively purchase OS X Lion for others, but also provide those without a credit card with a means to purchase content from the Mac App Store and Apple's other digital stores.
Numerous sources have indicated that Apple's retail stores are planning "overnights" tonight in which staff will change out promotional materials and update other aspects of the store layouts. Tonight's activities are presumably related in large part to OS X Lion, as the stores have reportedly received hard drives containing the new operating system. Other reports have claimed that Apple will indeed be releasing OS X Lion and new MacBook Air models tomorrow.
Update: The Macitynet.it report has been pulled by request. We have also received word that the Euronics location has removed its prematurely-posted Lion signage.