Slow progress is being made with tiles from Virtual Earth, Google Moon & Mars, and custom Venus images hosted in a generic Silverlight Map Viewer -- panning kinda works (in addition to zoom)! It's clunky, tile-at-a-time panning (no smooth drag metaphor, yet) but it (mostly) works.
Earth (VirtualEarth tiles)
Moon (Google Maps tiles)
Mars (Google Maps tiles)
Venus (custom tiles)
UPDATE: 'earth' view doesn't seem to work in IE. Not sure why... give it a go in Firefox.