Ross just posted something about the changing sense of community on Many-to-Many that prompted me to think a little further about the definition of "community".
It's like trying to define the word "party". How do you know when it is a party? What makes something a party vs. a gathering or an event? In the end, it only matters if I think it's a party.
Like "party", community is something that each person feels on a different level. It may be across blogs, in a wiki or message board, or all of the above.
It's not a "community" because it's happening on a message board, for example. That's like saying a gathering is a party because it's happening in a ballroom. It's not the location, it's the attitude of the participants.
Like a community, the party host/designer has the job of creating the appropriate environment for a party to happen. Whether it actually happens or not is up to the participants- if they think it's a party, it's a party. If not, it may still be fun and productive, but it may not be a "party".