I was not able to find any sections in NEC that prohibit switches from being installed in closets, so until somebody can point to a specific section I'd say it's not a problem. There may be adaptations to the code in your local area, so you'll have to check with the local Electrical Inspector to be absolutely sure.
The NEC does, however, have a section on luminaires in clothes closets.
NEC 2011
410.16 Luminaires in Clothes Closets.
(A) Luminaire Types Permitted. Listed luminaires of the following types shall be permitted to be installed in a closet:
(1) A surface-mounted or recessed incandescent luminaire with a
completely enclosed lamp
(2) A surface-mounted or recessed fluorescent luminaire
(3) Surface-mounted fluorescent or LED luminaires identified as
suitable for installation within the storage area
(B) Luminaire Types Not Permitted. Incandescent luminaires with
open or partially enclosed lamps and pendant luminaires or lampholders
shall not be permitted.
(C) Location. The minimum clearance between luminaires installed in clothes closets and the nearest point of a storage space shall be
as follows:
(1) 300 mm (12 in.) for surface-mounted incandescent or LED luminaires
with a completely enclosed light source installed on the wall above
the door or on the ceiling
(2) 150 mm (6 in.) for surface-mounted fluorescent luminaires installed
on the wall above the door or on the ceiling
(3) 150 mm (6 in.) for recessed incandescent or LED luminaires with a
completely enclosed light source installed in the wall or the ceiling
(4) 150 mm (6 in.) for recessed fluorescent luminaires installed in the
wall or the ceiling
(5) Surface-mounted fluorescent or LED luminaires shall be permitted to
be installed within the storage space where identified for this use.
Best Answer
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.