Graphics chips with built-in WebM support are slated to ship this quarter. It's not hard to imagine that the 2012 iPhone hardware will get this ability for free, even if it's only a side effect of using the same graphics chips as Android devices.
Then it's just up to Apple whether to turn it on. Google appears to be laying the groundwork it can now so that the answer is more likely to be yes.
I hope I can be forgiven for interpreting what you said as a statement, instead of as a true how-to question. My statement in reply was that this will take a few years to play out.
Chrome's support doesn't make an immediate difference to you or to content publishers. Firefox doesn't support H.264 <video> either and has larger market share. So rest assured that no soldering gun is suddenly needed, and you can continue to use H.264 on your iPhone 4.
Google's groundwork is supposed to come to fruition years from now, when you find an iPhone 5 or 6 already in your pocket.
The FSF is calling for a strategic direction that helps their ultimate goals, which you may respect and share. But if you slavishly followed their edicts, you wouldn't have an iPhone in the first place.
a) may be able to persuade your vendor (Apple) to support webm in your device,
b) for the time being, you are going to watch low-resolution, low-bitrate version of youtube. All higher quality streams will be webm and you can return to point a).
Google can, therefore, flip a switch and turn everything to WebM. They just need broad support in place first.
If Google says they will switch YouTube to WebM gradually, then all the browsers and devices will need to support it. End of story.