I've played with KDE 4 a bit, and it looks like it steals Apple's font antialiasing algorithm... at least for Konqueror. It's really pretty.
The nice part is that it works fine with xmonad, which is something I couldn't give up. I used to take great pride in perfectly arranging my windows. Complete waste of time, it's much nicer to have the computer do it for you.
Ubuntu starting with version 7.10 kicks OSX butt when it comes to font rendering. The default setting is not-so-awesome (I suspect to cater to folks transitioning from Windows, since it's a bit Windows-like) but if you configure it to slight hinting + subpixel smoothing (I suspect this is what KDE does), it destroys everything else. My few Mac-loving friends (and even wife) agree.
Linux font superiority isn't as noticeable on English alphabet, but with other languages it especially shines. That's the power of free software for you: people in those countries made sure their language is getting premium treatment so they worked with FreeType guys, while Apple had to deal with "priorities".
I haven't read the source or followed along with development... but I think it's a coincidence. Everything on OS X uses the same font rendering engine, and everything in KDE 4 uses the same font rendering engine, but those same font rendering engines are different. So I think the algorithm was borrowed from OS X to Qt 4.
I could be completely wrong though. Konq is the only KDE 4 app I use, and I'm using the super-experimental Debian pacakges. I also don't use it heavily. (Too many firefox extensions :( )
The nice part is that it works fine with xmonad, which is something I couldn't give up. I used to take great pride in perfectly arranging my windows. Complete waste of time, it's much nicer to have the computer do it for you.