The My Phone beta team notes in its blog (and WMExperts reader JoeG notes in our comments) that Microsoft's My Phone beta service will be down today from 7 to 10 a.m. PDT (that's 10 a.m. to 1 p.m. EDT) for maintenance.
They're also pushing out an update (just log-in on your phone and you should see it) to "address some known issues." The blog notes that you need to install the update within seven days of downloading it or My Phone will quit working for you. But don't panic. You can still get it online at myphone.microsoft.com/install.