Yeah, mine only works on 3G. If I start off on wi-fi, I have to turn off it off , let it connect to 3g and then, restart the Xoom. I don't have a Verizon phone and, the only way to use "MY Verizon" is to request a password through the mail.
With that password, I assume I can log into "my Verizon" while still being on wi-fi. I found this out when I first owned the Xoom and, when I got my password, I had returned it already. It took so much to get the reps to understand what I wanted the first time, I don't feel like calling them and going through that again, at least not right now.