Of course, there are sites with more than 550,000 nodes. For example, at Sunnyvale Drupalcon, someone was talking about an (internal) Drupal system used at a defence contractor, that had a node for every single part in each of the jet fighters produced by that manufacturer. They were talking several million nodes.

I know of a few other sites that are under development, that have these sorts of numbers. Usually where data is mass-imported from some other source/database (like geographic information, a product catalog, book indexes, etc)