Repli is a simple freeware SMS auto-reply!

Features:

* Shows who it has replied to. Uses the contact picture if available so you can see at a glance. Useful if you’re using this while driving and you are curious who texted you but you don’t want to be distracted.
* Unlike other programs of this nature, this one does not interfere with the text in anyway. It still goes to your normal inbox. It still gives the ringtone and vibrates like usual. This is by design.
* Custom messages. Save your list of custom replies in the settings window, or simply type one into the main window quick if you’re in a hurry.

http://forum.xda-developers.com/showthread.php?t=691924