One of the best replacement SMS messaging apps for the iPhone is biteSMS. It does much more than the inbuilt Apple app.You'll have to have a jailbroken phone and you'll find biteSMS in the Cydia app store for £5.95.
You can have the app pop up on your lockscreen - or any screen for that matter - whenever you want to send a text message. And when you receive a message it pops up where ever you are on your phone and gives you the options of saving, replying or deleating. It actually does much more than this, like make your camera LED flash when you have a new message. You can try it for free for 15 days.
If you want, you can also sign up to buy biteSMS credits to send cheap text messages. But, if you do't want this, you can still use the app without this service.
Update: biteSMS 5.0 is now live on Cydia
Comments
The final release of biteSMS 5.0 (now in Cydia) DOES work with Inelliscreen. You can choose in bite settings to use Activator for swipe-in gesture for Quick Compose. This temporarily disables Intelliscreen when the Quick Compose box appears. It was a feature that wasn't present in the beta version of biteSMS 5.0.
Hope this helps.