Implementing identicon using canvas uncovered some issues relevant to developers. I'll list them here for discussion:
No Data
Storing data used to render into a canvas directly in canvas is a common use case IMHO. With identicon, I am using the 'title' attribute on canvas element to store the identicon code but it would be nice to have an attribute for this purpose ('data'?).
No Script
Since canvas can't be drawn into unless javascript is enabled, canvas support should be disabled if javascript is disabled. As it is now, fallback doesn't kick in and canvas area appears blank.
Spaced Out
I worked around the No Script issue by wrapping a fallback image inside a 'noscript' element but, since the blank canvas takes up space, both will show next to each other when script is turned off, screwing up layout. While CSS tricks can be used to overlap them, I think unnecessary complications should be avoided if possible.
Active Fallback Unnecessary Get

Firefox fetches resources referenced from canvas fallback content even when canvas is working. My expectation is that it shouldn't do this because, when large number of canvases are used in a page and each had fallback content, page would load very slowly.