A chat a few weeks ago at our OSCON party with our friends from Krugle got me thinking again about the ability to search across multiple community sites -- essentially getting search results from communities other than the one you were searching from. Companies could agree to be available to/from different communities through an opt-in affiliation process.
My guess is that there is a good cross-section of our customers (and sites with other platforms) that would be interested, but implementation would raise some questions: Would you have to go to the other site to read the full thread? What information could be exposed via the search? Could people on one community discuss threads in another? RSS? Web services?
Given that companies invest a fair amount in their communities, the implementation needs to protect that investment and shouldn't lead to content ownership issues like the Flickr/Zooomr situation. It should instead serve to a) get questions answered more quickly, b) get people involved in other communities, c) avoid redundant questions on different communities and d) provide a richer dialogue.
It's a nice step towards interoperability, open standards, etc. We have had some good discussions on it in the past, and I would love to see those discussions get pushed further in the coming years.