Could always use an iPod Touch, if you have one.

That's actually my current setup. It still kind of defeats the purpose though; I have to be logged in to Google Voice in order to authenticate logging in to Gmail with the same account. I'll wait for them to get a little more creative before I expect it to be much safer.

Or a iPad v2 or 3.

Two factor auth works fine without a cell phone. You can put a land line in as a backup and of course you should store the extra physical codes somewhere safe as well.

Obviously though, this would be a pain to use without a portable device that can generate the appropriate time-based code.