A brief article on the Bookseller website mentioned that solicitor and tech consultant Justin Patten has set up a wiki (a website on which anybody can add to the content) to help him write a book about blogging and social media from the legal perspective. (If you want to have a look, it's here.)
The book will be published by Gower Publishing. Gower's Jonathan Norman said, "As a specialist business publisher, it is really important to know that the content will deliver what the market wants."
It will be interesting to see the extent to which people participate. There is no obvious payoff, but then there is no obvious payoff to contributing to the best known wiki, Wikipedia. However, that's a non-profit venture for everyone, whereas here Patten will be making money from his book.
Call me cynical, but I have a hunch that to get a significant number of people to participate, you'd have to give them some incentive. It could be a lottery--everybody who takes part gets a number and at the end there is a drawing and the winner gets a prize valuable enough to be tempting; or at the very least, the names of all contributors would be listed prominently in the book and they'd get a copy, or...?