Does Chrome really have the market share required for this move to have any effect on the decisions of web designers?
Yes. Chrome is rapidly eating market share: in just about 2 years since launch, it's at 13.5%. This is twice the share of Opera and Safari combined. But the decision to drop H.264 doesn't put Chrome "versus the world", as they already had Firefox and Opera in their camp (which also lack H.264). Opera + Safari + Chrome make over 50% of the browsers used today, in market share.
This is substantially different than the previous situation, where Google, Microsoft and Apple all had a H.264 browser, and Firefox looked like the odd one out, while Opera was quietly awaiting the market to decide (they'd have no choice but support H.264, if Firefox did it).
However, the battle is still not over for H.264. The common wisdom is that Google is pushing their WebM standard and that's why they drop H.264. If they really think it's that simple, they have not done their math right.
The growth is with mobile devices. The leaders among them is Apple with iOS, and Google with Android, both of which come with hardware support for H.264, and no WebM hardware support (future support in... theory, but I can say, count Apple out). So what are web content owners left to do? Maybe encode all content twice: WebM and then H.264. Imagine the hassle of, ironically Google's very own, YouTube, having YET another version of every single video they have in their library: FLV, H.264 and now WebM.
No, actually web authors will opt for the simplest choice, that's least amount of work: the same H.264 video everywhere, making use of hardware support for H.264 in mobiles, exposed via HTML5, and ... Flash on the desktop, which also support exactly the same H.264 videos.
So, in attempt to push WebM, Google may end up accidentally (or not..?) cementing Flash's position on the desktop as the video player for the foreseeable future.
I used to think Flash will considerably fade away once IE9 becomes mainstream (which comes with GPU accelerated renderer and H264 support), but now things are suddenly interesting again for Adobe.
A very insightful look at the issue. Thanks to the big mobile device market, this move would not harm H.264, but will help flash.