Hopefully this is helpful to you, honestly not sure if I read this or just experiemented and it worked.
If I found the post where I read this trick I will definitely credit them add the link in the future.

Tip: when tweeting use www. instead of http:// saves 3 characters

Not sure why the shortening services have not picked this trick up yet.
Linkee is not automatically doing this either.

Its possible that a www. link does not work in a client's like Tweetdeck or Tweetie
If you try it out in those clients and it works - please comment here.