I had heard about a year ago that T-Mobile made Google pull all of the tethering applications from its site. You'll first want to check with T-Mobile to see if "tethering" (aka "DUN") is even possible with a G1 on a T-Mobile account.
Provided you can manage tethering via T-Mobile, the next issue is getting your G1 to support the DUN Bluetooth profile to make the TomTom connection. Android @ 1.5 (aka Cupcake - what a weenie name for a release, eh?) had some bare bones DUN support. How far have you progressed with your Android releases on your G1?
I think you have your pastries confused.
1.5 = nickname cupcake
1.6 = nickname donut
2.0/2.1 = nickname eclair
There's no such thing as a "cupcake" other than version 1.5 of the code, and there wasn't much in the way of support for tethering at that point. Heck, they just finally got around to a BT profile that would allow file transfers in 2.0.
If you're going to do a "ROM" replacement, you'll really want to know where you are and where you're headed before you proceed.
So what version of Android do you actually have?
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