Not to mention the fact that webkit itself is a fork of khtml - so while apple's contributions are clearly valuable, it's quite a different story. If apple had not open sourced webkit, given its origins, that would have been resoundingly bad PR at the time.

Apple built on oss foundations, and most of what they've open sourced is simply a result of them not closing forks of once open projects, which is very valuable, but nevertheless less impressive that releasing important bits of their own making.