Microsoft My Phone page now has a sign-in button which lets you get on the waiting list. It may be US only, not sure. But it is first come, first serve. (Sorry for the late notice; we’re on vacation this week.)
Microsoft My Phone pre-registration
As we launch Microsoft My Phone, registration is limited and being made available to new users on a first-come first-serve basis. Please indicate if you would like to be notified (by e-mail) when you can sign-up for the service. We appreciate your interest and patience.
Add to wait list
Or
Sign up using promotional code
Use your Promotional Code to sign up and continue with your registration.
Thanks to Sebastian V. for the link.
UPDATE: I should provide a link back to JCXP, which originated this story.