The latest update to the geocaching-centric Magellan eXplorist GC has given it the ability to accept National Geographic Topo and Digital Globe aerial imagery content. The GC has no expansion slot, but does have 500MB of memory available for added maps, etc. This doesn’t turn the GC into a great hiking unit, as I believe it still lacks some significant navigation features, but it should expand its capabilities for both urban and backcountry geocachers.
If you want to learn more, I recently tested the NG Topo maps with the eXplorist 510. And if you pony up for new maps for your GC, let us know what you think!