If you are planning to migrate from a Mac using Snow Leopard to a Mac using Lion you will need to get this update. Here are the release notes.
This update addresses an issue with the Migration Assistant application in Mac OS X Snow Leopard that prevents transfer of your personal data, settings, and compatible applications running Mac OS X show Leopard to a new mac running Mac OS X Lion.
This doesn’t matter if you are just going to be installing Lion on a new Mac though. It is only important if you are going to be transfering information from a Snow Leopard computer to a Lion computer. Seems we are getting ready for launch.