IIRC it's so since Titanium PowerBooks which were the first to go widescreen at Apple, so it's 2001. Then aluminum PowerBooks with 16:10 screens, too. Intel MacBook Pros continued the trend.
... Wait, really? Huh. I thought the Powerbooks were 4:3 til the end, but yeah, just looked and you're right. (I had a G4 iBook at the time, which was 4:3).
