A PDA version of Bridge Builder would be the most incredible thing ever. It wouldn't need fancy 3D graphics. It wouldn't really even need color. Just bridges and physics.
It would be an incredibly addictive game that could be taken anywhere. It would make people miss bus stops. It would make checkout lines stagnate. The world would slowly grind to a halt to play Bridge Builder anywhere, anytime.
Or Gish for PDA. Gish would be pretty cool, too.