For some reason a development profile seems tied to the computer as well as the specified phone on the iPhone. What this means is that when you move the device from one computer to another to work on some software (say, a personal laptop to a personal desktop and back again), you need a way to clear out the development profile before things will work again.
I learned this the hard way yesterday when I blew away the contents of my iPhone, only to have things not work correctly (and deleting all my music at the same time).
Turns out the profile certificates are accessible and can be deleted from the iPhone itself: under “Settings” navigate to “General”, then scroll down to “Profiles”: this will show a list of profiles installed on the device which can then be deleted. Delete your development profile(s) from the device before plugging it into the new machine, and Xcode will load the profile for that platform onto the iPhone and things will just work again.