I've read that this was a change made midway through the 07 model year production, so some 07s have the change, but some do not.

Also, I've read that if you want to have a key with the remote controls integral to the key rather than as a separate fob, the 08 Fusion key will work on the 07 Mustang, but not on the 05-06.

 

This is correct. The Integrated Key/Fob only work on 2007+. The 2010 Mustang will come with a similar one standard. The only difference between the one I have and the 2010 one is that you have to push the trunk release button twice to open the trunk on the 2010, but they both look the same.