If you're thinking of a pneumatic piston, you might want to do some calculations regarding how much force you can get from one, and how much air it takes to get that much force over the distance you need to lift the robot.
It might be more feasible to get the job done with a motor driving a winch.
We don't have a design yet, but we have a lot of ideas. Here's one that a student came up with and made a model
http://www.youtube.com/watch?v=V254FdHQnyE