USB doesn't offer the bandwidth needed to handle even DV (SD) editing. And even though USB2 has transfer rates that rival FW, video streams needed for editing are not supported.
USB will work for DV. But not consistently. Oddly enough USB 2.0 works just fine with Avid and DVCPRO HD footage. I'm using that now. I swear it SHOULDN'T work, but it does. Well, things are SLOWER than if I had FW or eSATA. But USB and FCP, with anything over DV, is problematic.
I frequently edit on a 2 year old MBP over USB 2.0, editing HD footage (DVCPRO HD) and it runs fine, even at multi stream. Annoying pauses randomly, and of course it's faster on a tower with an eSata RAID drive... but it's fine and far from "hopeless"