Lon mentioned the two I would have mentioned (I like Locktopus myself), but it's worth mentioning that biteSMS has a nifty built in "secret text" feature for those messages you don't want anyone to see. It's nice because locking it down with Locktopus or Lockdown kinda gives the impression that you have something to hide, whereas with the built-in biteSMS feature, the texts you choose to hide just don't show up...until you tap a certain area and enter a password.
The BiteSMS hidden messages privacy setting is false security. The messages will still show up in the stock messages app. And hiding the stock messages app won't help as it can still be accessed via Spotlight. Locktopus would be better, but it can be disabled via SSH.
Did you try opening the Messages app that comes with the iPhone and seeing if the hidden messages show up there? I know they are hidden in BiteSMS, but I'm wondering if they are also hidden in the stock messaging app. If not, I think it's a useless feature.