Well, I sort of liking the WIKI idea too.. Let me ramble a bit...
One of the complaints I had originally and I still see new arrivals mention it some times, is that while some of the offline books do good job of being an "intro to Arduino" there is a gap between those documents and what is online at Arduino CC.
I feel that most of the GAP is related to what is in the Playground (In my opinion) and also what is in the forum as "solutions" to problems.
A lot of these solutions could become wiki entries... as part is a more useful "frequently asked questions" section, for example.