It was a just bit more than a "slight" increase. I think Apple had been planning on being able to double it all along - hence the initially lower dpi. But of course they'll never say for sure.
I meant they accomplished a slight increase in DPI across the genre (not relative to the iPhone 3GS) -- a large number of phones that came out months earlier are within spitting distance of the iPhone 4's DPI, but it just was never a big deal.
