First time caller, long time listener: I've seen this question asked on a few other sites but with no "definitive" answers.
The Apple website on iPhone batteries is ambiguous, at best, and the closest it comes to answering the question is saying that a monthly deep cycle is a good idea. (reference: http://www.apple.com/batteries/iphone.html(reference))
So, what do you think -- is there an adverse impact from repeated, overnight charging of your iPhone 4? Does this negatively affect battery performance over the long term? Or is there some chip/ software that acts to shut off the charging once the battery is topped off, thereby protecting your hermetically sealed little power source?