You need this OS X Update to install OS X Lion.
Apple released what is to believed the last Snow Leopard update today with OS X 10.6.8. The release notes of 10.6.8 make it clear that you have to update your Mac with this update in order to download OS X Lion from the App Store and install it.
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From the OS X 10.6.8 Release Notes:
- The 10.6.8 update is recommended for all users running Mac OS X Snow Leopard and includes general operating system fixes that enhance the stability, compatibility, and security of your Mac, including fixes that:
- Enhance the Mac App Store to get your Mac ready to upgrade to Mac OS X Lion
- Resolve an issue that may cause Preview to unexpectedly quit
- Improve support for IPv6
- Improve VPN reliability
- Identify and remove known variants of Mac Defender
Clicking Software Update now on your Mac will find the 10.6.8 update. OS X Lion is going to be released in July. Apple did not reveal an exact date yet.