Honestly, invisible edits are a UI problem that people really ought to solve - highlight (or in some way indicate with a color change or such) that something has been edited (and specifically, what has changed vs what is unedited in the comment).

Or perhaps restrict editing, e.g. instead of being able to delete text, you can only strike through+slightly shrink+grey out text.

Not that I really like Facebook, but that's one thing they handled well with their comments. You can click on the edited link associated with a particular comment that was edited and see all versions of the comment.

So if someone edited their comment three times, you would be able to read all four versions of it.

I hope you relly rely lik typos siting there forever.

I'd take typos over that any day.

Plus they don't sit there "forever" because the archiving is usually crap.

Also it's IM. If you're going to obsess over grammar and spelling maybe just use email.