Keeping a rechargable device plugged in after it's been fully charged.

Factual Questions
1

There are plenty of times when I will need to charge something, and it will not be convenient to sit next to it for an hour and thirty minutes waiting for it to beep or give some other indication that it is charged.

Sarcasm aside, I may want to plug something in before I go to bed (or work) and feel sure that it’s had enough time to fully charge. Yet I recently got a new phone (Nokia n73) and in the instructions it says leaving the phone plugged in for longer than the charge period (or after it’s charged) can shorten the battery life.

If this is true of all batteries it’s especially worrying of devices without removable batteries such as my MP3 player. I might want to plug it in at night and fall asleep.
What’s the deal? How seriously does leaving a device plugged in affect battery life???

2

Here’s one take on recharging batteries which discusses the concept of “trickle charging” and related concepts. As I understand it, trickle charging depends more on the charger’s ability to sense a full charge than on the battery, but I could be mistaken.

3

I thought most modern portable devices - particularly mobile phones - managed the charging process themselves - with the phone’s operating system controlling the hardware. I have to wonder if these clauses/warnings in the instructions aren’t just there to give the manufacturer an ‘out’ when the battery life inevitably shortens anyway.