I turned on my four-year-old iPhone 4 last week for the first time in almost two years. The home button is definitely losing its input responsiveness.
Oddly enough, my six-year-old 3G still works a little better.
Heh, I had the same thing. By the end of it's use, the 3G I had even had a cracked back from when I dropped it, but the buttons were fine.
iPhone 4 home button gave up. Thank god it was jailbroken at that point so I could gesture to the homescreen.