Just wanted to let ppl know I've had the same problem other users have been reporting. My "last batch" Touchpad arrived in the mail sometime late August, and had been working well I guess, although the center button was never functional. I figured I wouldn't really need it and having dealt with HP tech support before, I decided to just accept that I was stuck with a minor defect that didn't significantly affect my user experience.
Well, today my Touchpad began booting and then switched to USB mode, with the usb icon plastered on screen.
The solution is to hold the center and power button down for 15 seconds or more. Unfortunately, since my center button was non-functional out of the box, I'm having to send it in to HP.