I've noticed something clever in twitter (or twittermail - havn't worked out which).
If I create a mail message with a tinyurl url it gets passed via twitter unaltered to my twitter page. If I do the same thing but use our in house short url service, which is based on nano url, the resulting twitter display is a tinyurl link.
Odd. Actually if you look at the flow what has happened is that the non-tiny url has had a tinyurl created for it - implying that rather than truncating the message there's intelligence to process and automatically generate tinyurls for all non-tinyurl url's