Just one more thought: It can't hurt to call your lender and ask if they will accept a lower pay off at this time. However, be sure to have it in writing that this will not affect your credit, and that the loan will be marked as Paid in Full.

Even if they do agree, I still strongly recommend you speak with your accountant or financial planner first.

I agree with Dan - the bank has no reason to accept a lower pay-off than what is owed. In fact, by you paying it off early, the bank is losing the anticipated interest income on your loan.

I would recommend you speak with your accountant or financial planner to determine if paying off your mortgage is in your best long-term financial interest. Sometimes it makes financial sense to keep the mortgage, utilize the mortgage interest deduction and use the funds you have elsewhere.
If that is the case, then it may make sense to refinance into a new, 15 year loan at today's rates.

Why would they do that? With all of the money banks are losing now due to short sales and foreclosures, I don't think you have any chance at all of negotiating down your principle balance on a loan that is current, for someone with the assets to pay it off.
For a bank to approve a short sale, an owner has to prove hardship, and a COMPLETE lack of any other assets.