As we mentioned in our what Mac owners need to know after today's WWDC announcements roundup, Snow Leopard (Mac OS X 10.6) will be required for Lion.
If you are using Leopard (10.5) or less, you must first upgrade to Snow Leopard.
Amazon shows it backordered 2-5 weeks, (available from some 3rd parties), but the online Apple Store has it for $29USD plus shipping. Note that if you are running a version of Mac OS X before 10.5, you should buy the $129 Mac Box Set
