Web developer Chris Black benchmarked an HTML5 animation made using the Canvas tag against the same animation running in Flash. He tested HTML 5 running the same animation on an iPod touch, iPhone 4, and a Nexus One, but also ran it in pure Flash on the Nexus one. The results may surprise you:
HTML5 Canvas on iPhone 4G: 22fps
HTML5 Canvas on Nexus One: 40fps
Flash Player 10.1 on Nexus One: 57fps
Oh, and Flash Player used half as much battery.
Now, does this mean that HTML5 sucks or shouldn't be used? Of course not! As Chris points out in the comments, it may be possible to tune this Canvas implementation to run better on mobile. He also points out ways that Flash could run better on mobile.