I have yet to see a compelling reason in this entire thread to NOT have one. The only reasons i have seen is A: I dont want it, which isnt a reason thats an opinion and B: It would mess up the looks.
For B: I ask any of you to take a look at an HTC EVO. It has a notification light and its behind the speaker grill. Since you know its there you will probably say that would bother you. Well a little known fact is that the EVO actually has TWO LED notification lights. One on the left side of the grill for notifications but also one on the right side. Hardly anyone ever knows that one is there because its not being used. It was put in there to notify of a Wimax connection but never made it into the final software. So to me that tells me that Apple COULD put in an LED that could be turned off and no one would know it was even there.
IMO its the biggest feature missing now. Especially if they allowed custom colors for notifications (ie red for email, blue for text, green for missed calls etc)