The Mustang appears hanging from the ceiling in both the Museum of Technology and in the Capitol Preservation Society in Rivet City. There are no actions associated with the airplanes and are simply there for decoration. They can be damaged, but unlike most vehicles they will not explode, being pre-atomic, and the wings will instead separate from the fuselage.
The P-51 Mustang appears only in Fallout 3.
Behind the scenes
Based on the appearance of the cockpit canopy, the comparative size of the air scoop intakes, the lack of inner wing 'kinks' and the tall height of the vertical stabilizer, the P-51 Mustang shown in game is the P-51D model or later.