Hi Tim, as I mentioned on radar, I do not believe that Web 2.0 is about user interfaces, but rather making it easier for applications to understand other applications.
For this reason, I dont believe that Flickr is a Web 2.0 application. A Web 2.0 application is one that does a small task and does it well, then it is re-used (a bit like using standard libraries in development). A great example of this is Salesforces' AppExchange.
I describe more of my point of view as well as examples in my post What Does Web 2.0 Mean for Business (http://www.nik.com.au/archives/2005/10/26/what-web-20-means-for-business/)