Actually, Apple released Migration Assistant Update for Mac OS X Snow Leopard 10.6 yesterday. From Apple’s site:
About Migration Assistant Update for Mac OS X Snow Leopard
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.
So! Be sure to update before you install Lion 10.7! Either update via Software Update under the Apple Menu on your machine, or download from Apple’s site.