This is a tiny hint that hopefully will save the rest of you some time. I have been using a Motorola cell phone (V300) to connect to the internet via GPRS -- it's actually T-Mobile's Internet Service, which is quite a bargain at $20 per month for unlimited usage.
The thing that is important to remember is that when you configure your cell phone with Internet Connect, it saves the OS X USB modem device string literally from the first port you plugged the phone into. This means if you plug it into the second port later and try to use the phone, it will appear to be working (i.e. the phone appears on System Profiler), but it will refuse to actually make a network connection. The way you can tell is to open Console and look at system.log. You will see references such as pppd[240]: Device '/dev/cu.usbmodem1B11' does not exist.
If you open Terminal and look in the file system, you can see that you have plugged the phone into the wrong port: in my case, I will have a file called /dev/cu.usbmodem3b11 (note the 3 instead of the 1). The way to solve this is to unplug the modem and put it in the original port; then the modem will be at /dev/cu.usbmodem1b11 and all will work again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050806203553192