What is SMS? From the application page:
"With Mafia Wars SMS alerts, you can now get notified through a text message on your mobile phone the next time someone attacks you, your mafia member needs help, or various other actions.
In addition, by being signed up, you'll get other benefits such as exclusive promotions and access to mobile only content. Finally, all actions are tied with your Facebook account!
*Your Mafia Wars account will be credited with 20 reward points 30 days after service activation."
In other words, if you subscribe for SMS, you will get your in-game news feed sent to your cell phone via text message. There is no additional charge (aside from your own cell phone carrier's rates for text messages).
It seems as if you can help other players on jobs via the service in addition to participating in other social options. You can also filter what news items you want to receive, so you can turn off notifications for common events such as getting attacked.
In the future, there is also supposed to be "exclusive" SMS content (that is there will be more perks down the road for SMS users). On one hand, I want to wait for this exclusive content to pan out before putting my cell number in. On the other hand though, 20 free godfather points is quite enticing!
Source : MWU