Why this hasn't been done before is a mystery to me. This little hack project by Kellbot of NYCResistor takes a rolling ball, an optical mouse, a gutted PS2 controller, and some Arduino hacking and makes it into a working (and awesome) virtual Katamari. Why didn't they have special controllers like this that came with the game? It makes so much sense!
The idea is surprisingly simple if you know how the PS2′s analog controller is wired and have some circuit-building know-how — requirements which leave me out. But if you're crafty, and have access to a giant steel ball and three smaller ones, you might be able to rig this up since the design is open source. It's essentially just a giant trackball, but it's not like you can just whip one of those up anyhow.