Higgs:

It probably isn't the hardwood at all.

Here's how they build a house:
First you have the floor joists. Then they put 3/4 inch plywood or 1X6 lumber boards over the floor joists, and that's the "subfloor". Then they build the walls of the house over the subfloor. Then they put thin plywood or particle board "underlayment" down over the subfloor in each room. Then they install the actual flooring (harwood, carpet, ceramic tile) over the underlayment.

Normally, when floors squeek it's because the subfloor is rubbing on a nail that's supposed to be holding the subfloor down. What happens is that if they put the floor joists in when they're still too wet, the floor joists shrink in size, and as they shrink a gap opens up between the subfloor and the floor joists. The squeek you hear is the nail and the subfloor vibrating as they rub on one another as you walk over that area of the floor.