This was an interesting topic. I've used larger wikis before for quick overview information or as a starting point on tough reference questions. I don't usually give Wikipedia info out as solid fact, but it's good to know things like the approximate date of a battle to look up more information with a history question. I've found that sometimes the information is more specific than what I can find elsewhere -- that always makes me wonder what the author's source was.
I added a link to the MN 23 things on a stick wiki - the resource page - to a similiar program that's running in WI libraries. Of the wikis listed to look at, I thought the Princeton book one was the most interesting. I can see it's application in even a small library or for an associated book group to give out recommendations for others to read.
Although I work in a public library, I know that some of the teachers in town will not accept wikis (or, in a few cases, any websites) as resources for projects. I can see the validity of both sides of this stance ... they want the kids to learn to judge the accuracy of information rather than just writing down the first facts they find; however, sometimes wikis can lead to finding great new sources of data AND some wikis do include traditional source notes.