We uncovered some other large geo-sites using Joomla including SanDiego.com and TimesSquare.com but I have a new favorite: NewOrleans.com.
Back in 2006 the site was rebuilt in Joomla by a company called Silicon Chisel who gave a detailed rundown on the move. Not all of those pieces are still in place but the core of the site is still Joomla and I think its a very effective, professional set-up.
The template appears to be very loosely based on Synapse from Rockettheme.com and extensions used include: