Hmmm, perhaps there is more work to be done. I know we had no problem creating five prestige classes for our forthcoming Guardians of the Underhalls sourcebook. Of course, the sheer difference between surface druids and deep druids made that exceptionally easy. As I read over the rest of the forums though its appearing that there may be more than a little good information in there for those who play surface druids.
Prestige Classes from Guardians of the Underhalls
Dark Walker - A malevolent druid who turns from the art of wildshaping to embrace the darkness that pervades his native caverns. Sneak attacks and darkness related abilities, as well as continued spells (but not caster level for companions)
Deep River Tracker - A druid in tune with the vast rivers and seas of water deep beneath the surface. Continued spells and caster level for companions compliment this perfect underwater combatant.
Shifting Guardian - The supreme advocate of the wildshape, this druid forgoes his tune with nature to master the art of the shapeshift. Massive shifting focus with spells every 3rd level. No caster level for companions.
Sporemaster - This druid takes his love of fungi to the next level in an attempt to become one with it. Nasty offensive and defensive capabilities (such as DR/edged) are coupled with continued access to spells but no wildshaping or animal companion levels.
Underearth Stalker - This druid stalks the underearth, restoring balance at his leisure with a combination of rogue abilities and unique combat abilities. While access to animal companion advancement and spells are delayed (every other level), they do allow the stalker to continue his relationship with nature.
Well, thats quick and off the cuff, but I think you get the idea. I love the druid class myself, having played many a druid (including a shapeshifting planar druid) in my day. Maybe I'll see what I can compile and add to this fall and see what I can come up with.