Web rendering engines are extremely complex. There are very few companies that have the expertise to create and maintain one on their own. (Again, the similarity to Linux is strong here.) I’m glad all those developers at Apple and Google are working on improving the same open-source web rendering engine, rather than dividing their efforts between two totally different, proprietary engines.
Ever since Opera announced that they would be switching to WebKit, some have bemoaned the loss of their Presto engine. While Opera has a market share of around 2%, it was the principle of aligning yet another browser with the Apple-Google behemoth. These fears, as Siracusa articulates beautifully, are misguided.
By the way, if you want to know about all the little differences between the various WebKit implementations, Paul Irish has written a smart (and thorough) article.