No need to tether Wifi... Bluetooth tether works
I have a droidX, and was syncing the two devices to push a file across them when I saw bluetooth tethering... it works flawlessly. I never even heard about this during all the "screw the data plan, I'll wifi tether my tablet" hoo-ha.
Basically: sync up a phone with the tablet so they see each other.
Then go into bluetooth settings on the Xoom: settings > wireless and networks > bluetooth settings > paired devices > (YOUR PHONE)... and click the wrench icon. There's a tether option right there sitting in broad daylight... no need to even have a rooted handset I guess?
works great for me.