I think there's a bit of confusion with many fans because he has not yet "officialy" signed, but it's pretty much a definite, as Fox has already stated that they won't do a movie without him. He only says to the press it's not "definite" to give himself leverage while negotiating a contract, I think.
I think sites like this are the sources of the stories/rumors: (delete the space after "http:// " because KMC wouldn't let me post the links otherwise).
http:// www.killermovies.com/x/xmen3/articles/3039.html
Read the "Wolverine note" on this site:
http:// movies.yahoo.com/shop?d=hp&cf=prev&id=1808490829