FYI...there is a bill that is stuck in either the House or Senate (can't remember which right now) in which one of the provisions is to make PMI tax-deductible. It may take another year or so, but it should be here eventually.
It's a slightly different ballgame with FHA loans that require very small down payments (3%), thus the MIP is the cost of putting down so little for down payment on a house.
It really depends on the difference in the interest rate of the second mortgage vs. the interest rate of the first mortgage as compared to the PMI rate.
Tax deductibity is really a function of how much house you buy and what the interest rate is. Our interest rate was low enough on the house we bought in 2003 that we don't exceed the standard deduction.