The USB Control Device Class Abstract Control Model - CDC ACM
USB CDC (Communications Device Class) ACM (Abstract Control Model) is a vendor-independent publicly documented protcol that can be used for emulating serial ports over USB.
Neo1973 and CDC ACM
The Neo1973 u-boot boot loader contains support for obtaining the boot loader prompt over USB CDC ACM.
The OpenMoko kernel also contains CDC ACM device support, contained in the g_serial.ko kernel module.
Host PC and CDC ACM
Linux has a generic CDC ACM host-side implementation in the kernel module cdc_acm.ko