My wife is having this issue. Very annoying...she keeps hanging up on people because of it.
I am hoping it is a software-related issue; my guess is that it is.
When is this OS 4.01 update coming out?!
Its happening to me too, except im hitting the mute button leading ppl to believe I hung up on them. I am also hitting the contacts button so when I go to hang up, I am on the contacts list. I remember the iPhone 3G had this issue but a software update corrected all of it.