Apple man Chris Marrin recently revealed this news on the public WebGL mailing list. "WebGL will not be publicly available in iOS 5," he said. "It will only be available to iAd developers."
On the desktop, WebGL is supported by the stable version of Google's Chrome browser, Mozilla's Firefox, Opera, and the nightly builds of WebKit, which serves as the basis for Apple's Safari browser.
Presumably, Apple is testing the technology with a small numbers of devs, before rolling it out to a larger audience. But iAds is at least an amusing choice. ®