[IKE]

How often do you need/want the arm to move?
How heavy is the arm and the load it will move?
Will the arm only be in 2 positions or will it need to stop in intermediate locations?

If 2 positions, then pneumatics could be a really good answer, but you need to be mindful of how many times you want to do the operations and pay close attention to your storage volume calculations. With the unlimited allowance of on-board storage (from a volume perspective), you have a lot of volume you can work with, but recharge is still limited to a specified compressor.

Motor solutions for arms allow for more felxibility, but often require a fair amount more engineering. One mistake I often see with teams is using the window motor for an arm. It has high torque and is essentially self-locking which makes it seem like a natural for an arm. However, it is low power which makes it really slow. The motors also have thermal protectors which keep the motor from overheating, but also keep them from functioning at critical times.
Instead, using a high reduction gearbox and some additional chain reduction can get you the high torque with a higher power set-up (quicker movement with higher loads). FRC 33 2005 and 2007 have some nice shoulder driven arms. Check "Behind the design" for 2007 for more details.

For arms with a small range like you are discussing, I really like the geometry of the HotBot 2007 (FRC 67) or Robo-wrangler 2011. These arms use a push rod and a lower pivot that drives the arm up. The geometry can also be set up so that the linkage will "lock out" a vertical height which can alleviate the stall concerns.

If you can answer my initial questions, we can start to work through some solutions.