Currently HARP is not eligible for investment properties. As Rodney indicated below, the new HARP program is set to come out soon (most lenders are nervous about offering this product).
Imagine how difficult it will be to sell a loan to an investor and having to show proof that the house is upside down in value. Most lenders that offer this will have to be ready to get hammered in foreclosures if the economy doesn't turn around. Obviously without the ability to sell the loan, they'll have to absorb the loss.
I have access to a lender that allows us to choose the appraiser instead of using the bank's management company who assigns someone who may not know how to give you the value you seek. You'll need 20% equity in the house to do this.
If you are waaay upside down, contact the lender where you have the mortgage and see if they'll streamline the loan for you. Sometimes going Wells to Wells or Chase to Chase does the trick.
Worse case is that you stay put and write off the interest at a higher rate until congress does away with that deduction.
You can check out freddiemac.com or fanniemae.com for updates on their programs.