Not on a "Verizon" branded Xoom you can't, Google Voice or anything else...yet! After the major "4G" upgrade early this summer, SMS and MMS along with the micro SD memory slot will be enabled and added to the Xoom services. Those services are disabled as of now so there is NO way to text or mms from a "Verizon" Xoom using the Xoom itself or the Xoom's assigned phone number!
My work-around for the time being is a great FREE program called Text-em. Its web based, sends and receives text and mms messages, and uses an email address or the services inbox for retrival of messages...Cheers!

