I've used my T-Mobile GPRS service as the backup for the corporate laptop for a couple years now. The instructions for that were clear. I just today got it working on my Mac Laptop. The key is the modem scripts and configuration information at Barkman's GPRS page. I had found the scripts when I first got the laptop, but the instructions were missing. I couldn't get it to work. Now that there is the nice big table of configuration parameters and better instructions, I got it working on the first try.
This is very useful so Barkman deserves the google love of a reference.
I also like the T-mobile service for it's low price. I can usually find WiFi, so I don't want to spend a lot of money for broadband. This service costs only $30/mo, seems to be genuinely unlimited use, and the lower speed is good enough for email sync and lightweight browsing. But the T-mobile web site and help desk were completely useless and unable to help configure a Mac.