Be careful with this one. There are the federal requirements, Freddie Mac has their own rules and so does Fannie Mae (no they are not exactly the same). Also, in Minnesota we have a state PMI law that is different as well. So, depending on if the loan is in-house or sold, and then depending on who it is sold to, makes a difference in which rules you follow. Also, whether the original loan was made before or after the 7/29/99 date affects the automatic termination feature.
I have a cheat sheet I put together for our Loan Servicing Department. It might help (no guarantees of course). If you'd like a copy, e-mail me at firstname.lastname@example.org.
This is a very touchy spot for consumers and you need to have all your ducks in a row before dropping or not dropping PMI.