Just to clarify, it's a microSD slot, not SD.
Verizon reps have been saying they will enable it when the 3G Xooms are sent in for the LTE upgrade. The Motorola site says a future software upgrade will enable it. If you have the wifi only Xoom, it will only require the future software upgrade.
In the future, spend a few seconds on a search, there have been a few threads within the past few days on this issue.