I've got agree with Brad D. I couldn't discriminate against something they made that was to do with Twin Peaks. They didn't have to write the Access Guide and Coop's book, and keep Laura's Diary in the family but clearly they wanted to have quality control over it so yeah, I say it's canon.
Oooh. I reread this post and realized I sounded rather snooty in my original post. Oh well.
I still firmly believe in FWWM's inadequacies, and do actually believe it to be a poor film now, but I have changed my ideas about all other TP related things. After reading the three books, there are of exceptional quality and add very much to the series. They are definitely cannon.
The card series was super fun and besides the birthdates, are cannon.
So yeah
Sometimes things can happen just like this. *snap*
Even if Laura's diary isn't considered canon it should be because I think that made me care about Laura Palmer much more than either the series or FWWM. It's wonderfully written and probably my favourite piece of tie-in media to a show ever. Not read Cooper's book yet, but I will soon.
I wouldn't go so far as to call it "canon", but I always feel that Mulholland Drive is so thematically related that it forms part of a greater whole.
I'm pretty sure that the woman in Silencio is not actually Sheryl Lee, but it looks so much like her, and it seems so appropriate that she would be, that I'm actually content to wilfully believe that it is. I view it almost as a what-if scenario of what might have become of Laura had she not died. I think there are quite a number of other connections between them - Welcome to Canada, Dad's best friend...
In some ways, I've even thought of them as part of a trilogy - Blue Velvet, Twin Peaks, Mulholland Drive.
to me i regard it all canon even if it has inconsistency's and i think what would makes fwwm interesting is how it opens up new questions that could have been answered in season 3 like laura knowing about dale being trapped and the ring.