I recently replaced the hard drive in my girlfriend's iBook. After removing the old drive, I put it in a USB external enclosure. While I was installing OS 10.4 onto her machine, the Migration Assistant appeared. It allows you to transfer file from another partition or another Mac via FireWire. However, it says nothing about USB. I plugged in the old hard drive via USB and tried it just for kicks.
So despite the apparent lack of support, you can transfer your stuff via USB as well as FireWire. Hope it helps.
Mac OS X Hints