I can confirm the Xoom (and most android devices) don't work with Ad Hoc networks.
I can also confirm that using a jailbroken iPhone4 with MyWi you CAN tether using bluetooth.
I followed these directions to get bluetooth tethering to work:
1: on XOOM, go to settings>wireless & networks>tethering and portable hotspot and make sure that "Bluetooth Tethering" is UNCHECKED, this is NOT what we want
2: on XOOM, go to settings>wireless & networks> enable airplane mode
3: on XOOM, go to settings>wireless & networks> enable bluetooth
4: on your android phone, go to settings>wireless & networks (or equivalent)> enable bluetooth
5: Make sure that both devices are discoverable, so that they can see one another, should be in settings>wireless & networks>bluetooth settings
6: on XOOM, go to settings>wireless & networks>bluetooth settings>select your phone in order to pair it with your XOOM
7: a box should pop up on both devices, select Pair on both devices
8: on XOOM, once the pairing is complete (still inside bluetooth settings), select the phone, it should say paired but not connected underneath it
9: The two devices should connect, and if it works then the bottom right hand corner of your XOOM should have two bluetooth icons, one should indicated that your bluetooth is turned on and the other should say bluetooth tethered.
Note: That I had to use MyWi to setup Bluetooth laptop tethering NOT the Bluetooth on demand option.
Hope this helps.