Ensuring unique account logins across the globe
Travis: Can't find the link, but flikr recently had a post somewheres (maybe highscalability.com?  I cannot find it for the life of me...) about how they handle unique IDs.  In essence, they have one "ticket server" that only runs a single database, with a single table, with two fields -- `id`, and `a`.  The `a` column doesn't really do anything, but it allows them to issue a mysql REPLACE INTO, which just increments the auto counter, without expanding the table size.  Then they have a mechanism to retrieve this next ticket.Found the link -- does this concept work for you?
http://code.flickr.com/blog/2010/02/08/ticket-servers-distri...