As discussed in Should we be allowed to edit comments, one simple solution is the digg.com comments way. I don't like digg.com nor use it but their method is quite an intelligent way of allowing you to edit typos.
Basically when you post a comment, you are given 30 seconds (or it might be 1 minute) to change that comment. After that the comment is locked and cached, and you can only delete it.
It sounds annoying to code, but could be a good compromise between the current system and plain out deleting?