The same thing is done in the online version of Real World Haskell and is used fairly effectively by the readers to share solutions and clarify concepts.
It is used effectively. Of course, programmers being programmers, it's also used to point out errors and typos. I've also read a bunch of good comments about choosing clearer wording for some parts.
If I was writing a blog (and I'll start one one of these days :P), I would definitely want free, crowd-sourced editing that way!
http://book.realworldhaskell.org/read/getting-started.html
I believe this input was used in revising the book for the print version.