Disney runs over 1400 instances according to this presentation: http://www.10gen.com/presentations/mongosv-2011/a-year-with-...
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