I've been a member of Flickr for quite a while, although I haven't uploaded any pics of my own yet. I joined because my nephew has an acct where he posts pics of his family, and I subscribe to his RSS feed so I know when they have new photos of his 4-yr-old up! (She's quite darling, btw).
While I can think of myriad creative ways that Flickr might be used in the classroom, in large part I think this discussion is moot in that I can not see local school districts in my very conservative area of Texas allowing students access to this type of site. Ever. Concern over inappropriate pictures and discussions is just too great for my district to take that "chance," in my opinion. In fact, in many a workshop, we have been reminded that anything posted or saved for viewing by the public on our network should be "appropriate for even the youngest of our students" to view. Since we have 4 year olds in our PK programs, I suppose that means that even high school students' work should be appropriate for 4 year olds if it is to be published on our network or on the Internet in some way. Ludicrous.
So. Flickr is out for our schools, I should think--even if it is a tool that our students enjoy and find creative (and even appropriate) uses for. I do think it's worth our time as educators and 21st century learners to learn about such technologies--they're cool and useful! I don't see us having the opportunity to use them in the schools so that we have an opportunity to take advantage of those "teachable moments" and guide our students in using them--within the curriculum--safely and ethically.
As for 3rd party mashups, the guy who maintains FD's Flickr Toys has a really interesting blog as well, where he blogs about his family as well as his work designing software. Some of his blogs about being a dad are quite touching. He is a very talented photographer as well--his flickr page & blog are worth a look!