WebGL is not supported by any mobile browser yet (http://caniuse.com/webgl)
(edit: it seems that Blackberry Browser and Opera Mobile support WebGL. My bad, but it is not possible to use these browsers on iOS)
If you can read caniuse's table, you'll see that WebGL's actually supported on mobiles. You can get a more quantitative view from http://webglstats.com/ if you'd like.
Blackberry has been pretty active in the WebGL community. I was actually surprised and impressed with the performance the first time I saw a demo on the Playbook!
WebGL works in all browsers on desktop and on most smartphones/tablets - except on iOS platform (Safari, Chrome) as Apple has disabled the support [1].