Unless you've built that float-down into your loan, lenders don't really care if the interest rate drops while you've got your rate locked. They may look at it if rates drop substantiallly and quickly, and don't appear to be fluctuating back upward (substantial is 3/8% or more.

You do have other options. If you've gone directly to a bank, you can go to a different lender and start the process over again. You may not have to go all the way through with this, because just looking at another bank gives you some leverage over your original bank.Let your original lender know that rates have dropped and you're considering your options; they may be happy to work with you rather than lose you to a competitor.

If you've taken your loan out through a mortgage broker, they may be able to easily and quickly move your loan to another bank with more favorable rates. You should ask about it.

Some lenders lock you into an interest rate only on a specific property. That means that you can't lock your rate until you have signed a purchase contract. Other lenders, however, will make a lock-and-shop program available to you, in which you can lock in a rate before you find a home. When rates are rising, this can be very nice; but this will almost certainly cost a little more than the prevailing rate, and the lender may charge a non-refundable fee or a deposit toward closing costs to ensure you do use their loan.

If you're building your home, lenders generally offer long-term locks for a higher fee and an up-front deposit. Locks for these loans can be as long as 180 days or more, but will cost a point or two more. Most of these locks do offer a float-down, so shop around until you find one.