Can an ipod be powered by mains ?

I'm thinking about getting an ipod although it will be mainly used for connecting to hi-fi and not carried around much. I can see that the 40GB ipod comes with a dock that connects to the mains, but is this solely for recharging the battery, or will it power the ipod indefinitely too ? I know the ipod has battery issues and I don't want these to get in the way of mains operation. Anybody know ?