iPhone 4S Home Button Flush
The fastest way to get a quick read would be to take it into any of the retail stores. You could check with 4 or 5 floor models and something like that is pretty easy to feel by setting a few phones down on the table and getting two or three people to compare them.

It's likely something small is stuck inside the switch area (like a grain of sand or smaller) or the spring got physically depressed more than it should.

You might get it swapped if the feel of the button is different or at least get it seen at the bar and have the genius write it up so there is a history of it not feeling perfect on the service record. (just in case you want to call in for a repair later over the phone)