Recently, I started looking at Canvas on Android, and I came across the C5 Canvas Benchmarking test. I tested it on both the Motorola Milestone that I recently bought, and the Sony Ericsson Xperia X10, and it was pretty surprising. Now, it should be noted that the Motorola Milestone has a TI OMAP3430 processor, and is capable of speeds up to 600 MHz. However, the Xperia X10 has a Qualcomm Snapdragon processor, and has a clockspeed of 1.02 GHz. Now, people have been speculating whether this clock speed will make a difference, and I think that when you look at the results, it clearly does.
Now, I don't think that performance is quite there yet, but you may see more advanced uses of Canvas on mobile web applications in the future.