It really destroys the flow of reading though. These light-boxes should be more passive. I agree with the first commenter. It's super annoying and something nice becomes ugly.
I still find the animation of it appearing distracting and I think it interrupts the 'flow' of reading the article, but it was easy enough to block the element with my ad blocking extension.
As an aside, Firefox + Noscript are a thing of beauty. Using them I just don't see most of these annoyances. Yes, it 'breaks' many websites but this usually means a much faster page render without all the annoying stuff.
Unfortunately slamming a SIGN UP NOW!!!!! box in the middle of the page gets more people to sign up than a "Please sign up over here" box at the side.
The problem may be a lack of systems thinking.
If your metrics are sign-ups, then the aggressive pattern wins.
If your metrics are committed sign-ups by users who are genuinely interested and aren't simply going to ignore your emails for the rest of eternity, the picture may be more complicated.
Too many UX people don't seem to think this one through.