One little-remarked-upon feature of iOS 4.3 is that it's now possible to tether your iPad to an iPhone. And not only over wifi (available on the iPhone 4 only), but over Bluetooth as well.
It's pretty simple to set up. Switch on tethering on the phone, and allow it to enable Bluetooth. Then, on the iPad, go to Settings -> General -> Bluetooth, and select the iPhone. A funny symbol will appear in the top-left, and you're connected!
Why might you want to do this instead of wifi tether? Well, first, wifi tether is only available on the iPhone 4; I think that this will also work on the 3GS. Secondly, of course, it's highly likely to take less battery power than wifi tether. There's increasingly little reason why anyone might want to buy the 3G version, though if you really want to be able to browse the web over 3G for 10 hours at a time, it's probably still the only option.
Update: Apparently this also works with the iPhone 3G, and at least some non-iPhone phones.