1. Apparatus for bone surface-based registration of physical space with a tomographic image, comprising: an ultrasound transducer generating ultrasound signals representative of a distance from said transducer to said bone surface as said transducer is scanned over but spaced from said bone surface; tracking means generating transducer position/orientation signals tracking transducer position and orientation as said transducer is scanned over said bone surface; means synchronizing generation of said ultrasound signals with generation of said position signals; physical model generating means generating from said ultrasound signals and transducer position/orientation signals a three-dimensional physical space model of said bone surface; means generating a three-dimensional image space model of said bone surface from said tomographic image; means generating an alignment of said physical space model and said image space model; and output means generating an output utilizing said alignment.