Just got an email response from Lookout: "Hi Matthew,
The reason why you're having trouble finding Lookout in the market on the Xoom is due to the fact that the Xoom doesn't support SMS messages. We rely on SMS messages to initiate a connection to a device in the event it's lost, stolen, etc. Without the ability to receive SMS messages the missing device features would be greatly hindered, HOWEVER, device based backups and malware protection would work normally.
If you decide you still want to download Lookout to your Xoom you can head on over to getjar.com and install it through there. You just need to perform a search once you get there.
Thanks for using Lookout!"
Makes sense.