This is an open question, but it is a logical followup of my previous question asking for an emoticon keyboard app.
It appears it is not possible to define a system-wide keyboard that supports ASCII emoticons. Instead, specific applications can provide a custom keyboard or another way to easily input emoticons.
So what replacements for the SMS app do provide shortcuts to input ASCII emoticons? For use by people unfamiliar by all those weird punctuations.
The app should not require a jailbreak.
Bonus points for apps that also display ASCII emoticons graphically.
UPDATE: Now iOS does allow system-wide third party keyboards, so this question is no longer really relevant. However I keep it alive because the answer "no third party app can do SMS" is still correct and useful.