I haven't tried it yet but there are several references on the web to using a laptop to bridge the ethernet and wireless connections. Here is a link to one of them:
One of the ones I read said that the ethernet cable needed to be a crossover cable but the other references didn't mention that. They are bridging to game consoles so maybe one brand needs the crossover and another doesn't.
UPDATE: I went ahead and tried bridging with my Win7 laptop. Went into Network Sharing Center / Change Adapter Settings / select both wired and wireless connections / right click on one of them and select bridging
A new icon should pop up that says "Network Bridge". The ethernet connection from the laptop to the Tivo required a crossover cable.