Just tried the Froyo USB tethering with Ubuntu 10.04 and it's frigging awesome. I put this under Hacks because this is with a Droid using the latest Froyo leak. I'm sure it will be the same for the final releases.
Here's how to set it up:
1) Plug your Droid into your computer
2) Under Settings -> Wireless & networks -> Tethering click 'USB Tethering'
3) Click on the Network Manager icon in the system tray
4) In the menu that pops up click on 'Auto usb0' (although I'm guessing depending on what you have plugged in, the number might be different)
5) That's it
Unplug to disconnect and then uncheck USB Tethering on your droid.