If YouTube goes WebM, it's hard to imagine that Apple won't be forced to cave in on this one. And it's hard to imagine that Google wouldn't re-encode the whole of YouTube (geez, the CPU cycles!!!) But I would expect legacy support of H.264 by YouTube for some years to come. So even if Apple does switch their preference, it could be quite some time.
Apple already supports WebM as much as Microsoft does.
You can just install a Quicktime component that adds support for the container and the codec, which gets you <video> support without them having to do anything. Microsoft isn't shipping it either, but they announced that they're adding Google's DirectShow filter to their whitelist for IE9.
I'm very intrigued by Apple not being on this list. Considering that MPEG-4's container shares a history with Quicktime, and the amount of time and effort Apple has spent supporting h.264, I wonder what they're saying internally?
Interesting. With both Chrome, Firefox and Opera supporting it, there will still be plenty of options for viewing WebM on OS X though. I wonder what will happen to Safari if WebM gets popular?
Safari and Apple will have to cave in. If WebM becomes a de facto standard for video on the web, then Apple would be fools to limit their browser like that. Especially since it's patent free.