Cellular providers, including T-Mobile, will still charge you for the minutes/meg (depending on whether the plan charges you by the minute or meg) even if you're connecting to your ISP rather than their own Internet service. You really won't save money connecting to your ISP instead of the faster GPRS service T-Mobile provides (dialup connections are at 14.4k while GPRS averages 40 to 50k/sec)
-------------------- Lisa Gade Editor in Chief, MobileTechReview