I am having the same issue and am so glad I found this thread..(I was starting to think I got the 1/1000 lemon) See, this is my first ever iPhone but I used to play with my friends I4 which had a very responsive "clicky" home button. My 4S though feels very mushy. Now don't get me wrong, it registers but like someone previously said, it just irks me every single time i click it that unless I hit it directly in the center, it doesn't give me that 'clicky' responsive sound.
Now here's the weird thing, I took it to the apple store and they gave me a brand new model but sure enough, same exact thing..slightly mushy home button. I don't know if it's a white version thing or not but I'm about to go get another new one but in black. Sorry if I sound lame but for a phone I'm going to stick with for 2 years, I want everything to be perfect. A $500+ device shouldn't be "mushy" whatsoever.