If you've got a Mac running Snow Leopard and you're planning on buying a new Mac running Lion in the near future, you're going to want to update your Migration Assistant software.
The update fixes an issue that prevents a Snow Leopard system from migrating to another one running Lion. You definitely do not need this update if you're just upgrading one machine from Snow Leopard to Lion, so don't worry if those are your plans. If you're buying new hardware, however, this download is for you.
Migration Assistant Update for Mac OS X Snow Leopard [Apple Support via MacWorld]