The Twitter edit issue has been solved by either Slate or Salon (can't remember). Anyway, you have X amount of time to edit your post.

I'm guessing 99% of people wanting to edit a tweet do so to fix a typo, so a 1 minute timed edit feature would be perfect.

The reason they dont want to add it is they don't want people changing tweets after comments are made because the whole thread will get wonky.