I really hate it when sites ask for access to my email address book, and never allow them to do it. So I'm biased against that idea.

I much prefer a "popular with circle" approach, but as you say, it requires people to have some subscriptions in the first place.

What about something like offering a list of potential matches when people import their friends from other sites?

Something like:

"you've imported ironed_orchid@livejournal.com. There is a dreamwidth account with the user name ironed_orchid, why not check to see if they are the same person?"