Hello,
it seems, they have changed the system (for the worse):
Answer Center | AT&T
So, you 'll need an extra Sim for using it and can not use an existing (voice) sim.
The only place I found it, has been at Walmart.
Walmart.com: AT&T LG Turbo USBConnect 3G Prepaid Broadband Device: Cell Phones
The LG seems to be nice (but not working on the AWS T-Mobile UMTS network) and got even UMTS 2100 MHz. But it is network-locked and I do not know how (if possible) to change that.