While I find the parent overly optimistic (the day IE9 ships WebM is the day that I take a flight on Hog Airlines), I think this is also missing the point.
Hypothetically were IE to ship with WebM support, Apple doesn't have the market share to enforce their current codec monoculture. The converse is not true however; Microsoft doesn't care if Safari/iOS can play WebM. The onus is thus on the IE team, and I think Apple is irrelevant to the issue.
