No, but Apple is the biggest factor. I'm willing to bet more corporate decision makers have iPhone's than Androids (which also don't support WebM by the way) and when you're selling a video hosting and asset management service that matters.
These are the other extremely popular devices that support H.264 and most will not be supporting WebM anytime soon so why would I encode in WebM until they do.
Android devices.
Sony PSP.
Sony Playstation 3.
Xbox 360.
Blackberrys.
Some Symbian Phones.
Windows Phone 7.
I'll jump on the WebM bandwagon as soon as more then 50% of the popular consumer devices support it. Taking away support for an established and popular codec just makes you an asshole and will be nothing more then an inconvenience as long as Flash player supports it and is included with Chrome.