I admit, I really dislike sites that pull in contacts from an address book, so this is a really good, thinky question for all social networks.
Someone may have already suggested this (sorry, haven't had a chance to read all your replies) but I think you already have a section in a user's profile where they can put their ID on external sites (Twitter, Delicious etc). I think it's the 'about' section? It seems like that's designed to allow people to find you outside DW. If you make it possible to add other social network IDs (like LJ, Facebook, Google+ etc.) could those fields be searchable? Then a user could search for their friends by the username they knew on the other site?
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Someone may have already suggested this (sorry, haven't had a chance to read all your replies) but I think you already have a section in a user's profile where they can put their ID on external sites (Twitter, Delicious etc). I think it's the 'about' section? It seems like that's designed to allow people to find you outside DW. If you make it possible to add other social network IDs (like LJ, Facebook, Google+ etc.) could those fields be searchable? Then a user could search for their friends by the username they knew on the other site?