I prefer to nag people to not use shorteners at all if you're not on Twitter. I don't know if it works, but I feel that it's a habit that needs to stop.
Shortened links are annoying and misleading, I've been wasting too much time clicking links I've seen already and links to sites I just don't like.
So one to one mapping to SMS is not possible now.
When I enter SMS bigger then 140 bytes it's splitted transperently and the only consequence is increased payment for that SMS.
Also, users who actually use Twitter via SMS would probably not be capable of viewing the link anyhow so it could just say "(+lnk)" or something.
And thus uber-shortening will probably continue until someone develops alternative processes that are ~sexier~