Just an FYI, I managed to get this setup working on Sunday night:

C64 -> Turbo232 -> Airlink Raven XT (http://www.sierrawireless.com/product/airlink/ravenxt.aspx)

Using the Airlink as a normal hayes modem, I was able to "dial" out to telnet BBSes over cellular (In this case, T-Mobile). If you had a power inverter in your car or van, this could be the uplink for a mobile C64 rig.

While this actually works, the price point is kind of prohibitive. I was lucky enough to have a free unit from a demo, I think the real ones retail for like USD325 or so. There are cheaper alternatives, but I'd imagine their feature set varies. I tried a similar experiment with the LandCell 882-EDGE-GEN Cellular Data Modem, but it wouldn't dial out.

Since the ATD command on this device has been adapted for netcatting, you can also specify the port to use, making this a viable option for just about any sort of TCP/UDP client application.

Thanks for the update - eventually these may start showing up cheaper at surplus stores or whatnot.

I want a pic of you BBSing from the Sixmobile, or the woods