Most of the production deployments I find on the internet are around 3-5 nodes. Are there any production clusters that are running 500-600+ nodes?
Also foursquare runs a very large MongoDB deployment. http://www.10gen.com/presentations/mongodb-foursquare-cloud-...
Craigslist: http://www.10gen.com/customers/craigslist
Shutterfly also has a very large deployment: http://www.10gen.com/customers/shutterfly