A new freeware app makes tethering the G1 to a PC or laptop straightforward, without requiring that users hack the Android firmware or have root access. PdaNet allows a USB-connected Android cellphone to work as a tethered modem - with either cellular or WiFi data - with a single click.
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.
Of course, while this is technically possible, it could very well contravene the terms & conditions of whichever carrier is providing 3G data access for your G1. It's worth checking the small print to see what limitations on tethering there are in your cellphone contract.