Apple has open source figured out. It's the prefect fusion: the core technologies (http://webkit.org) are open source, but they manage the UI like no one else can. Software with style, the way it should be. The WebKit team is absolutely fantastic too, I miss them.
Yes, it started as KHTML, but Apple has taken it to a new level.
The KHTML and WebKit guys have had their differences in the past, but I believe KHTML is trying to merge all of the WebKit improvements back into KHTML, hopefully eventually unforking the two:
It's also worth noting that Apple could have used a different engine, or started from scratch.
Similarly, they could have chosen not to open source the darwin kernel, and they could have based it on something other than BSD.
The point was that Apple seems to know when to choose open source technology and leverage it, and when to start from scratch and build their own proprietary system.
