So Twitter introduces T.co. Well, if it still requires the "http://" then it's still too long for me.

What would be more interesting would be to introduce a new protocol, perhaps called "url"?

url://urlcode

url:// -- identifies the protocol

urlcode -- identifies the code of the url to look up

calling url:// would return results similar to the way http works.
A 200 code might return the full expanded URL.
Other codes would return error values or other info.

So, instead of http://t.co/xxxxxxx you would have url://xxxxxx. Half the characters.