Leaving a cell phone on the charger bad for battery?

Factual Questions
1

I have a Motorola KRAZR which I bought in August. Lately I’d noticed that the battery level started to blink soon after making a Bluetooth call for 30-45 minutes. So I took it to the Verizon store and said I wanted to buy a replacement battery.

They were nicer than expected and just gave me a new battery since the phone was only a few months old. But the tech looked at the battery and asked if I kept in on the charger overnight. Yes, I plug it in when I go to bed. He said the battery was “swollen” and leaving the phone on the charger when fully charged was bad for it.

Does this really make sense? I thought modern phone circuitry regulated the current from the charger to prevent overcharging. Is there really such thing as a swollen battery?

2

AFAIK, all modern cell phones use LiIon or Lithium polymer batteries, which require a smart charger. Therefore, leaving a phone on the charger when not in use shouldn’t harm it, and is what I do with my phone and computer.