I'm not a lawyer, but I've been following the legal issues surrounding this sort of thing for years, and I can tell you that there's not a damn thing Skinner can do about this. Posts from one discussion board are fair game to be posted on another discussion board, period. The only thing that would alter this reality would be if PJ-Comix was somehow personally profiting from the use of the posts (for example, by collecting DU posts into a book and selling them).
Sure, Skinner can get a lawyer to write a threatening letter, but that's just a scare tactic.
Yea, but that's my whole point. This is public information, willingly put out in the public realm by anyone posting on the DU. They've surrendered their rights to that material by the act of posting to a public Internet space. PJ has every right to use that material in his/her's creative venture, either in book or comic book form, poking satire at these people.
However, if such a book also contained comments from the humble correspondent following each post then it wouldn't be purely DUmmie posts and would alter that. An example is Al Franken's "Lying Liars" book where he quotes extensively from conservatives and then gives his twisted spin on the quotes.