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Um, not only is it insanely trivial easy to game, but it would have also been easy to "TweetRank" by counting the number of @-mentions of an account.

Absolutely, this is the technique used by Topsy/etc. That said, @ mentions have multiple contexts (replies, mentions, retweet, follow-friday, etc).

The formal retweet structure lets you build a true/formally defined social graph.

The graph, as you say, is only for one aspect (retweets). It would be neat if a tool can figure out the context and build multiple social graphs.