According to an interview with Comic Buyer's Guide (by way of Newsarama) David Benioff is working on a screenplay for the "Wolverine" spin-off movie, and he's writing it as an R-rated flick.
According to Newsarama, Benioff is a huge fan of Wolverine (from the comics) but also says that he can't imagine anyone other than Hugh Jackman playing the part.
His plan is to write it as an R-rated version that will be more gritty and rough than the X-Men movies, and then it will be up to the director and presumably Avi Arad as to whether the movie remains at that level or is dropped down to a PG-13.
I've read arguments going both ways: That a Wolverine movie should be rated R (he does have huge claws that he uses in battle), as opposed to PG-13 (or even PG, since he's a comic book character). Personally I think that a stand-alone Wolverine film should be R. If you're going to have a guy that slashes at his enemies with foot-long steel claws, there ought to be blood.
I've always thought that as a concept, the character of Wolverine always had to be watered way down in the comics. I mean here you have a guy with a dark, raging temper, awesome fighting skills and razor sharp claws that he's not shy about using... yet (as far as I know) rarely do any if his opponents die.
Don't forget that Marvel has put out R-rated superhero flicks before: The Blade trilogy.
Of course PG-13 equals way more box office money and it doesn't alienate the core fanbase of the X-Men movies: teenage boys.