I discovered this brief ‘howto’ (on twitter, of course) from @the_api_book via @ginatrapani.

Head over to google.com and enter the search phrase:

site:twitter.com/*/favourites your_username

where your_username is your twitter user name.

For example, this search returns the results for all of the folks who have favorited a tweet made by the user @lifehacker, and which tweet it was that was favorited.