I know if you unlock and root your device you loose your warranty. But what I would like to know is if you unlock your Xoom are you still eledgable for the 4G upgrade and is it still free?
I can't say for sure one way or another, but I would think you would still get it free.
The 4G upgrade is being marketed as if you have already paid for it in the price and you are just waiting for them to finish developing the hardware so they can install it for you. I don't see the 4g upgrade as a warranty claim.
The smart, best thing to do in this situation is to just wait until after you get the 4G update before trying to unlock your device. Supposedly it's only 90 days away.