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Slashdot has a link to an article that talks about the practicality of creating a single game world for millions of players.
http://games.slashdot.org/article.pl?sid=09/05/15/0357225
The question is whether or not such a thing would even be interesting. Do you look forward to operating in a colossal universe with millions of adventurers, warriors, pilots, jedi, archers, whatever?
I think I prefer the sharded universe where I can play with a couple dozen friends among a couple thousand strangers. A world with millions in it means that I have the same problems of anonymity and homogeneity that I encounter in life. One rule set for everyone. One storyline. One playing field. One place where the largest group can dominate the world's activities (EVE Online).
Shards offer the ability to play at my level with my prefered rules following my prefered storyline variation - should server-wide storylines ever actually come into being. All that is assuming that there are a couple thousand other people who like playing the way that I do and that a publisher is willing to devote a shard to our way of playing.