Apple has just released a tiny 768kb update to Migration Assistant for Snow Leopard, and if you’re planning on transferring your files and settings from a Snow Leopard Mac to a machine with Lion, you’ll need to grab this, because otherwise you’re out of luck.
According to the update 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 from a Mac running Mac OS X Snow Leopard to a new Mac running Mac OS X Lion.
Note that you don’t need this update if you just plan on installing Lion atop of Snow Leopard tomorrow: this is only necessary if you’re going to install Lion on one machine, then transfer settings from a Snow Leopard machine through Migration Assistant.