Those who have picked up a Motorola Xoom upon launch have noticed the absence of an extremely popular app from the Android Market: Google Voice. You can sideload it onto your Xoom, but it will crash when launching (see picture above).
The good news is that the app is currently being developed and optimized for Honeycomb; the bad news being that we don't know when we can expect to see it. Here is what Google employee Zeke had to say in the Google Voice forums.
Glad to hear from so many Xoom early adopters! As you've noticed, Google Voice isn't available for Honeycomb yet. We're working on it, and I'll update this thread as more info is available.