I think at the same time it's a problem in how we (perhaps the society) treat instant messaging. It's at a weird intersection between face-to-face conversation and serious textual discourse, so to say. So you're kind of supposed to be all friendly and casual and drop the first thing that comes to mind, but you can also get hanged for careless phrasing. So for the first case you shouldn't need a delete button, because everyone, including you should understand that you aren't writing a formal essay and mistakes are going to happen... On the other hand, it just stays there and it's possible to go over it verbatim and analyse your "subconscious" or whatever.

Also, what you say aloud essentially gets "deleted" a few seconds later, as it starts disappearing from memory, but IMs generally don't. I guess they do in certain chats.

So I would say we haven't really figured out online communication yet, and probably not any time soon.