UPDATE: Check out the new iPhone 4.1 Software update which solves this for good!
Been having a problem with your shiny new iPhone? Yeah, I did too. My face kept triggering the speaker button when I was in the middle of a call, almost every time! Now because I’m left handed, the speaker was my problem – I’ve seen elsewhere that people’s cheeks keep pressing the mute button.. I’m assuming those people are righties!