What happened to the Giuoco Pianissimo? Throughout a fairly long chess career, I have always regarded 4.d3 after 1.e4 e5, 2.Nf3 Nc6, 3.Bc4 Bc5 as a good beginner's move but not threatening, and I have felt sure that most strong players shared this opinion. Quite suddenly, 3.Bc4 and 4.d3 have become the choice of very many top players. In the recent Isle of Man tournament there were days when the top boards looked like a junior scholastic tournament (but with very few examples on the lower boards). I don't see a big difference between these games and typical 19th century games.

How did this happen? What has changed? Is there somebody out there who has been paying more attention than I have?
I agree with you. Players of all level play the Italian, and especially the 4. d3 line because they have read 2750+ players do so to avoid "drawing" (which is true at this level) lines aka Berlin Defense, as stated by Dag Oskar. 

I see a bunch of 1500 rated players playing these lines, shuffling around trying to get something out of it, with no understanding of what is going on. I don't really see the point to be honest. This is true that the Italian has some bite if you know what you do, but the Giuco Piano/Pianissimo tend to generalize way too much. Personally, I tend to avoid it from move one 1...c5 ;) because I want to play chess. I'll play 1...e5 again when the Italian will be out of fashion.