Apple’s patch notes say simply, “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 from a Mac running Mac OS X Snow Leopard to a new Mac running Mac OS X Lion.”
Note that Apple still hasn’t adjusted entirely to its new nomenclature of calling Mac OS X simply “OS X” starting with Lion.
Be that as it may, you can download the update through Software Update in Snow Leopard, where it is a 312K download. Apple also has a direct link to a standalone download on the KnowledgeBase article for the update, where it is a 714K download.