The topic was raised in the Motorola support forum, and a moderator replied in no uncertain terms that a rooted Motorola XOOM won't be eligible for LTE upgrade. This leaves you with two options if you want to hack the tab - not to root the tablet for the time being, or to unroot it when you send it to the service.
The Motorola XOOM is not able to offer its full functionality at launch, since at the time of its release, it didn't support Flash and lacked LTE capabilities. As we told you some time ago, if you want the LTE upgrade, you need to return your Motorola XOOM back to the factory, while Flash is coming in the next few weeks.
source: Motorola Support Forums via Android Police