I've seen this effect on the underside of some commercial DVDs I own. I haven't had any readability problems with these discs. :disagree:
Like you said, it's probably just a mark to identify where one layer ends and the next layer begins.
EDIT: I have a 5 pack of blank MKM 003's lying around somewhere. I'll check them and see whether they have that grain effect thing on them as described in the first post and post back the results here.
@adre - Does the grain effect come after burning or is it already there while the disc is blank?
I don't think you've specified this yet. :disagree: