The latest Fedora kernel has the ark3116 serial-USB driver for it. That driver is pretty primative: it implements no ioctls, can't do hw flowcontrol, etc. It was created by spying on the Windows driver using usbsnoopy. But it works, mostly.
There are two Linux tools that know how to talk to cellphones. Gammu and Gnokii. They both work pretty well. Gammu is the more complete tool, but it wasn't able to work over the ark3116 cable without a touch of hacking. I may contribute a patch back to the project, now that I have a lot more free time.
Also, GSM terminals can implement ETSI TS 127. Basically, you can issue Hayes-style "AT" commands over the /dev/ttyUSB0 device, and the cellphone respones like an old serial modem. Except that when you "dial" certain magic "phone numbers", it starts speaking PPP, and gateways IP over GPRS.
Except, it turns out, not all phones implement it. Such as, for example, mine. I have a Nokia 3120b. It does GPRS for itself, running it's own little web browser, and makes an IP stack available for JVM apps on it. But the greedy little thing doesn't share, it won't act as a GPRS gateway.