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Not so great - Seems to require your Twitter followers to associate their Google account on Migrat.us before it's of any use.

Yeah, and people can already link to their twitter from their profile page. So this shouldn't be needed, a simple crawler should be sufficient.

A crawler offers no warranty that the Twitter username is legit.

Scraping might not even be needed given that Google has an API for that http://code.google.com/apis/socialgraph/