I had a similar issue with my 650.

There's a known issue of the jack going bad, which can cause the problems with the built-in microphone not working. But mine developed a stranger problem--the bluetooth would work, but the built-in mic was intermittent, sometimes fading out during a conversation.

I can't really blame it on Palm, that problem started after I took it apart to replace the scuffed-up case. So I don't recommend taking it apart unless you are really, really good, since there are some ribbon cables and other things in there that can get nicked or damaged. It's pretty tightly packed.

Palm has a flat-rate repair charge for 650s that are unlocked or from Verizon, but says contact your carrier for others. An extra $25 will get you an advanced exchange with a refurbished unit.

I ended up getting a 680. Cingular has some good deals on them now if that's your carrier--$99.99 after rebate for a new one, or $99.99 out the door for a refurbished unit.

The 680 is a nice, but not revolutionary, upgrade over the 650. The battery life on the 680 is not as good as the 650, but I'm pleased with it overall.