From what I understand:
N1 is easily rootable by design (apparently an agreement between google and htc) but rooting places an unlocked icon on your bootscreen that voids your warranty if you have to send the phone back to htc. Currently this icon cannot be removed. Rooting the phone pops up an explicit warning to this effect.
Droid got rooted very shortly after the release of 2.01 (I think the update was specifically what allowed the means to be discovered.) Also, the droid can be easily unrooted, as it came about by exploit, not design.
Phones before that, I can't say. I'm fairly new to android myself. Maybe the next big update for the Eris will hold the key...