This is a simple demo object. It has gears on shafts inside a shell and would require internal support structures that would be pretty difficult to remove without support that dissolves.

Update: It worked! I've had trouble getting PVA to stick to ABS, however it sticks pretty well to PLA when you extrude both at around 210c. Here's a video:

http://www.flickr.com/photos/tbuser/6733182253/

Update 2013-02-04:

Also works with dissolvable HIPS filament supporting ABS. See: http://www.flickr.com/photos/tbuser/8522775217/in/photostream