Once you roll back to 2.0.1 and root, you then install a version of 2.1 that has root access*. You end up with the 2.1 upgrade (with its new widgets, live wallpapers, etc) and root access, which allows you to install Barnacle or the other wifi-tether.apk. It's actually quite easy to do, as is going back to the stock non-rooted 2.1, if one wished to do so.
*I'm not suggesting that 2.1 has been rooted (because it hasn't, from what I know) but for the purposes of being able to install apps that require root access, this works like a champ.