One of my heroes, 21-year-old technologist David Recordon (now of Six Apart and a leading light in the effortt to open the social graph) has written a letter to Apple that I endorse from beginning to end.
I have no problem bricking a phone if it was unlocked (I switched to AT&T despite hating everything that company stands for) or not allowing a firmware downgrade, but I do have a large problem bricking phones for no reason other than having third-party applications installed. The reason this affects me so strongly is that your company is now creating fear, uncertainty, and doubt in the minds of your power users as to if it is safe to apply an Apple update. Thus even if my phone is not designed to brick during the update, by bricking some phones the level of FUD is too high for me to be willing to update my own. You've shifted me from needing to be an early adopter to preferring to wait to see if I will be hurt by a software update; this is not good.
Tim Wu anticipated some of these issues in the best analysis of the iPhone to date.