a checklist of sorts:

1. extension mobility service must be enabled on ucm server
2. create a phone service for extension mobility under device settings->phone services
3. create a device profile under device settings->device profile
4. on the device profile, subscribe the profile to the extension mobility service
5. on the end user, add the extension mobility profile under controlled profiles
6. on individual phones, enable extension mobility. then subscribe phones to extension mobility service
7. login to phone with username and PIN

notes:
a mobility user can only be actively logged into one phone at a time.
if a mobility user has multiple device profiles associated with it, the user will be able to choose which profile they would like to use.
device profiles should be interchangeable across most phone models (despite differences in # of lines). however, i was unable to get a 7942 profile working properly on a 7940 and ended up having to create a separate 7940 device profile.

source:
http://www.youtube.com/watch?v=M50sykfNe5I