Gizmodo has just published an article that explains how to connect your G1 to a computer for data tethering.  The process requires a USB cable, the Tetherbot App,  the Android SDK, a little use of either Run (on Windows) or Terminal (on Mac), and it can only be used with Firefox. Carriers haven’t been too supportive of tethering, and you may want to either check with your own carrier for charges, or use this sparingly.

You can take a look at the step-by-step process here.