In his Rotacaster-Robot series, Xander Soldaat has created an Omniwheel vehicle that can be built with the pieces in one NXT 2.0 kit, and three omni-wheels.
A video, the NXT-G source code and LDD building instructions can be dowloaded from his blog (link).
A video, the NXT-G source code and LDD building instructions can be dowloaded from his blog (link).