I thought the idea was a virus would put some autorun nasty on your USB key while it was plugged in. Then the next time you insert it, whammo!! Like, some people leave the thing in for that ReadyBoost or RamBoost or whatever that Vista thing is that uses USB for performance enhancement.
And yeah, I usually turn off the the autostart stuff. Although in W7 I haven't bothered because it pops up a prompt rather than going ahead and doing anything. I've been closing the dialog. Of course that's my secondary PC so I don't use it that much anyway. Surfing unknown sites with Sandboxie on mainly.. and testing my apps.