"A very small number of Motorola XOOM users recently received an email from Motorola regarding the 4G LTE upgrade process. While we are very excited about being able to offer the upgrade this summer, and know our customers are equally excited about the benefits that 4G LTE speeds will bring to their Motorola XOOM, the email was sent in error. We apologize for any confusion this may have caused, and look forward to being able to share more specifics on how — and when — users can take advantage of 4G LTE on their Motorola XOOM in the near future."
If that email had only reached "a very small number" of customers, it wouldn't be such a disappointment. But we, and others, published the news to the larger XOOM population. So we apologize for getting your hopes up. The statement still indicates that the upgrade will come this summer, so hopefully you won't have to wait too long.
source: Droid Life