CDC ACM
From Openmoko
USB CDC ACM (Universal Serial Bus - Class Definition for Communication Devices - Abstract Control Model) supports serial communication (including AT-commands) over USB.
CDC ACM is used to access the U-boot bootloader prompt over USB (see: U-boot#Using_usbtty_from_Linux).
Specification
http://www.usb.org/developers/devclass_docs/usbcdc11.pdf (3.6.2.1)