See previous post on Matrix: PMI Gets You In The House: Now Get Rid Of It. 
The Homebuyers Protection Act  was passed by Congress in 1998 requiring lenders to notify homeowners when the equity in their home reached a level where PMI was no longer required.
Here is the testimony of Richard J. Roll, Founder and President, American Homeowners Association (AHA)  in front of the United States Senate Committee on Banking, Housing and Urban Affairs on February 25, 1997 on PMI. He was speaking about the abuse of PMI overcharges.
“Your home falls under this act if you purchased, constructed, or refinanced your single-family home after July 29, 1999, and your loan is not a government-insured FHA or VA loan. If you purchased your home before July 29, 1999, your lender is not required to cancel your PMI when you reach 20 or 22% equity, but many lenders will do so if you ask. “
Here is an article on the costs associated with PMI insurance to homeowners. 
There is significant incentive for a homeowner to get PMI removed from their loan payment. In order to do this you need the services of a certified real estate appraiser to provide a value estimate. If the home has appreciated enough to where the equity is at least 20% of the overall value, then the odds are relatively good that you can get the lender to remove the PMI.