Apple has released an update to Migration Assistant app on Snow Leopard to solve an issue for transferring your data from Mac OS X Snow Leopard to Mac OS X Lion.
Please note that this update is not required for running the upgrade installer to Mac OS X 10.7 Lion from Snow Leopard. Migration Assistant is used for transferring data between Macs. For example if you have a new Mac running Mac OS X Lion and wants to transfer the data from your existing Mac running Snow Leopard.
The update is available via the usual Software Update.