this happened to both my wifes and my treo 650. this has just recently happened to my wife's centro as well. I fixed it by blowing in the headset jack on her centro since i did not have a 2.5mm headset with me. I also tried unplugging and plugging the headset till i saw the icon change on the screen for the 650s.
So it looks like the problem is back with the new phone as well.
Oh well in other news i need a new cord for my headphones..... for when i finally find a pre that is
