I know that this could void your contract with T-Mobile but what the heck, there are emergencies where you need a way to get your laptop connected and if you can use your G1 as a modem why not. More information about the application here

Currently in beta stage, and available for Windows users (together with a 64-bit Vista version), the app reportedly works well. PdaNet is already available in Palm OS, Windows Mobile, BlackBerry and iPhone versions.

Via Android Community