The cheekily named TigerText — and I'll give you three guesses who it's named after (your last two guesses don't count) — markets itself as a way to handle your sensitive text messages. Want to send a little somethin' somethin' but you don't know if it'll spoil your future run for president? You can set up your messages to expire after a certain period of time, or even to delete right after the recipient has read them.
These aren't normal text messages you're doing this with, though. You're using special TigerText messages, and both you and the recipient have to have the app installed. The app itself is free, and you get 100 messages right off the bat (that you have to use up in 15 days). After you've burned through those with your, erm, mysterious texts, you can buy more for pretty cheap. The downside is it's a seperate pool of texts, though the upside is it works on devices other than the iPhone, such as the iPod.
Really, though, how discreet is something like this? Sure, your texts will be deleted, but the very existence of it on your phone will be enough of a red flag to cause suspicion.