While looking into the 3D printing world I came across a useful source of free 3D CAD software that is available on all platforms including ipad. It's free for non commercial use, and provided you share your designs with the world, it's even free if you decide to sell the stuff you make.
It's from auto desk, the makers of autocad. And there's a full suite of design apps to go with it, including circuit design and CAD/CAM software for exporting in gcode etc....
http://www.123dapp.com/design