Using the same semantics as toDataURL (image/png is the default output), add Blob getBlob([Optional] in DOMString contentType, [Optional] args...). Currently, an image must be copied to a string using toDataURL, then to an array buffer using base64 decoding, before it can be run through createObjectURL. getBlob would reduce the number of operations.
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-canvas-toblob http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2011-April/031243.html
Support for Canvas toBlob is likely to come from Chromium: http://code.google.com/p/chromium/issues/detail?id=67587
This API has been renamed to toBlob: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob
This was added at some point.