A new app for iPhone users is available on Cydia, the iReal Quick SMS. This app lets you access and write SMS no matter what you are doing on your unit.

It's kinda like QuickSMS... but not. QuickSMS gives you access to SMS with a swipe, while iReal Quick SMS does the following:

If your iPhone is Locked it shows a pop up on the Lockscreen with the keyboard launched.

From here you see a preview of the message right above the input box, You can go to sms app by taping on the Show Button or do the same by hitting Reply button.

If you are using the phone or are in another application, the pop still pops up and shows you the message in a small window.

You can also launch the app at any time, like QuickSMS, only for this app you hit the Value up button, which works even if the screen is locked. The app is available on the BigBoss repository.