I'm very happy to say that Windows Vista SP1 finally cured the remainder of the problems with ReadyBoost that I'd been seeing. For the last 18 months I've been unable to put my system to sleep at night because it would kill the ReadyBoost drive, without which performance would suffer. This means that SP1 has fixes for USB above and beyond the USB patch from last year.
For those wondering why I care so much about putting my system to sleep at night - it's the principle of the matter. Leaving lights on is throwing money down the drain. Leaving your computer on is too.