I leave mine connected all the time. iPod Touch 4th Gen.

The battery lasts what is it? 24 hours of continuous play?
It's charged when the car is on - or when I open the door, actually very nice feature - and when it's running. It never gets far below 95% or so. And it's in the center console so it's in a "cool, dark place."

The only caveat is if you leave it out where it gets very hot for long periods.
From an Engineering point of view, it isn't designed for that.
From ANOTHER Engineering point of view, much of it's slim profile is because there's lots of adhesives used in it (rather than mechanical connectors) and extreme heat can be an issue there. It will also affect battery life.

However, when the car is running, it cools off quickly (at least here in NC, mine seems to do that very well) and it's garaged most of the time, so those heat extremes are not so bad at all. I leave it on all the time - it may shorten it's life a bit, but the product cycle is so rapid, it will be time for a new BMW before the iPod Touch dies..



PS: It will also be way past time for a new iPod Touch by then.