The cradlepoint PHS with a cell modem (I have a UM-150 from Pantech with signal strength LEDs on the dongle).

There is a more subtle problem with the tablet's USB implementation - it may not be fast enough. Normally they would just use the ttyACM* or ttyUSB* drivers (or the airprime), and my cell phone and the cell modem are recognized, but after I tell it to start connecting, I get error messages which appear to be from data or interrupt overload.

And yes, the cell modem needs a powered hub inbetween.

Coming out soon is the Novatel Mifi which is a wireless router with built-in 3g modem. They seem to be supporting GSM/GPRS/3G as well as EVDO.