I'm working on a iPhone app that stores location data from a user. However, sometime the user doesn't have service.
Is there an API that estimates location data when the phone gets back into service? Or any other suggestions
No, there is no such API, because that would create wrong locations. You have to write yourself such a method, that hopefully works in the scope of your application demands:
E.g You could do a linear interpolation when the GPS service has an outage for some seconds.
e.g:
A liner interpolation of lat and lon values work without special geographic calculations.
Just it would not work if you cross the datum limit (border longitude = 180E to 180 W),
and maybe not if you cross the poles.
But both situations will practically not happen.