IF you've been syncing the 3Gs on a regular basis, including contacts to a supported application (address book on the Mac or Windows Address Book or Outlook on a PC) AND all of your media is in the iTunes library you normally sync to, AND all of your apps are in the iTunes library you normally sync to, then you should be able to plug the 3Gs in, back it up and sync it, Start up the 4S, go through the screens until you get the choice to Activate it and restore using iTunes, then plug it in. It should restore all of your apps, data and settings from the old phone to the new one.
BE CAREFUL. iOS 5 no longer displays the big swipe to cancel sync message when it's busy syncing with the computer. Touch the home button if the screen is dark and look at the top of the screen next to the signal strength meter. If you see 2 little rotating arrows, it's still busy.