Small electric actuator suggestions? Need something fast and programmable!

JoeNova:
Halloween is coming up!

I want to try and take a shot at making an arduino powered prop.
I need a small electric actuator of some sort. 12v is fine.
I'm having trouble finding something suitable because I have some particular requirements:
-Small. 8" or so is ideal.
-Powered by 12v
-Very fast moving. The quicker the better.

That is not a specification. Say what the minimum acceptable speed is.

-Programmable positions. Will need 6-10 positions with pneumatic like speeds.

Whatever that speed is... Say what you mean in numbers please.

-I only need a maximum of maybe 3-4" of travel.
-Will only be moving a couple of pounds.

force or inertia? or both? Moving a kg mass with rapid acceleration takes a very large force
and high power. Definitely need the numbers for this to see how feasible.