easiest way to do this is to download this file from your phone. it is the stock messaging icon you are looking for.
ic_launcher_smsmms.png
Open up your messaging app and then close it (this speeds things up a bit later in the process)
download shorter cut from the market (bettercut will also work, but i dont use it so i am not sure of the process)
make sure you have an open spot on the screen, longpress on the screen and add a shortcut
scroll down to shorter cut (better cut)
choose recent activity, then choose the messaging app. (if you didnt open the app before this step go to shortcut, applications, then choose it from the list of apps, this take a while to load all the apps and find the one you want)
then click change icon, choose image picker (
NOT the cropped one), navigate to the icon you downloaded (should be in the sd/download folder) and click on it.
change the name if you want to. and click create shortcut.
and enjoy the green icon