Script Summary: Automatically Squeezes tweet into less characters as you type using TweetShrink.com
Combined with Al N script Twitter Auto Replace URL with bit.ly URL the tweet sheds characters as you type.
One concern with shrinking tweets with tweetshrink --- search engines don't translate back so tweets may not be searchable... hmmm. I also observed that tweets in the live feed to tweetshrink were not shrinking by more than 10 characters. Anyway, good luck.
