I live in South Dakota and in 1994 (on July 4th to be exact) the 68th Strategic Missile Squadron was deactivated meaning that the Kilo, Lima, Mike, November, and Oscar Launch Control Facilities were deactivated (along with the 50 missile silos that they were responsible for).
Contact the USAF public affairs liaison for an accurate answer. They may allow you to tour if you ask.
You may also want to visit the museum at Ellsworth AFB near Rapid City: www.ellsworth.af.mil/museum.asp They may be able to give you more information on any other sites that are available for wandering around at without getting into trouble. :) Have fun!
It would be trespassing unless you got permission to be there. And trespassing on federal property is always a big deal. If you do decide to do this anyway please be careful. Let someone know where you are going and when you will be back. Really will not know the shape of the building inside until you are there. Sometimes floors are rotten in empty buildings, and there can be a lot of broken glass.
At the very least, it's trespassing. Edit: Abandonment does not mean you get the right to enter a building or other structure. If you have to go through doors or a gate, and you're there without permission, you're trespassing.
