Bravo for allowing comment editing, but what's the explanation for enforcing a five-minute window?
I think this might be over-engineering. If I have a spelling mistake or an inaccuracy I'd like to fix, I don't think the fact that five minutes has elapsed should prevent me from fixing it.
Further, if you are worried people will edit their comments in a way that makes the comment thread nonsensical, they can already do this by deleting their comment. So, limiting the ability to edit isn't really helping much in that regard.
Update
If people don't like this idea, I'm definitely open to hearing your reasons. For now, it seems like an unnecessary limitation of a quite awesome feature.
Update 2
Regarding John's scenario...
Me: Obama is cool!
You: You are right, man!My edit: Obama is an idiot!
You: You are right, man!
...this problem can still occur with deletions...
user1: Obama is cool!
user2: No, McCain is cool!
you: You are right, man!user1: Obama is cool!
---- deleted -----
you: You are right, man!
So, yes, editing could be abused as you suggest, but discontinuities can occur already because of deletions (and as someone else mentioned, because upvoted comments get prioritized upward).
As for Jeff's argument about rate limiting, I agree that is important, but I don't understand why you can't just use a CAPTCHA to prevent too many edits in a short period of time.
Save edits
. If they're going to put a timer on it, it should be visible and counting down while edits are being made. Also, a 10min grace window would be much more reasonable than 5, if there absolutely must be a limit.