From the fan page:
Coming soon – We’re bringing back robbing to Mafia Wars! Soon you’ll have to keep a close eye on your properties so they don’t become vulnerable. Get ready to pull some heists and bank all the new robbing collection items! Stay tuned!
Very cool. Unfortunately, it's not in the game just yet, and I honestly have no idea when it is coming. We heard back in December that it was "coming soon" and it is now still "coming soon", so there's no way of telling if it will be in the next few days or next few months.
It looks like they will be adding some sort of robbing collection. I'm guessing you will get a collection piece for robbing casinos, for robbing tenements, etc, until you have robbed every building type. I'm hoping you can raid other people's chop shop's for car parts though, that would be pretty nice!