The concern that I know of is that you don't want the wiring to risk shorting high voltage wiring to a low voltage wire or device. For that reason, you don't run phone and cable lines within the same wall cavity as electrical (and I also keep plumbing away from electrical).
It's hard to tell from the pictures, but I'd hope the USB components are fully enclosed, just as the internals of the outlet are. Assuming that's the case, and I'd be surprised to see thinkgeek selling it if it wasn't, then it's no different then worrying about the possible short when a USB adapter is plugged into the outlet.
Edit, looking at one of the zoomed pics I'm seeing what appears to be part of a metal enclosure around the USB, which should be grounded. That should prevent any shorting risk to the USB. That said, my concern is that the thin parts of the wall plate could easily crack on this, and then you're getting yourself into the task of creating a custom wall plate to fit the USB plug openings.
Edit2: As much as I love thinkgeek, you can pickup something like this surge protector/usb plug for less. No turning off of the breakers needed.