We should document more clearly that the count command may not work properly in a sharded environment. The count bug is SERVER-3645 which isn't due to be fixed until 2.8.

I think a warning should be added to the following pages:

http://docs.mongodb.org/manual/reference/command/count/
http://docs.mongodb.org/manual/reference/method/cursor.count/
http://docs.mongodb.org/manual/reference/method/db.collection.count/