The viral strategy needs to be a natural extension to the app that requires SMS - not sure you want to just use it for the sake of it being viral (seems forced).
A perfect example of this is the Twist app. The app tells someone you are meeting with when you will arrive. When you set up a twist a text message is sent to the person you are meeting with (if they don't have the app installed). The message says you are on your way and if they'd like to know when you will be there, download the app. It has been effective because it solves a problem, gives the end user an option and extends the usefulness of the app's original purpose.
Hope this helps!