Errors with Craft 2.3, Imagine and Imagick Does anyone else have the same issue?

Post the recent Craft 2.3 update all image transforms on my server (about 6 craft sites now) are throwing errors on new transforms. 

Specifically write errors from the Imagine library's Imagick classes.

My directories are writable and these sites have been working for some time. No new updates on the server, standard Debian lamp stack.
I've tracked it down / fixed it by explictly selecting my 'imageDriver' in the config. The docs say this is a new setting in 2.3.

It seems the default of 'null' is calling 'imagick' but not working, whereas choosing 'imagick' in the config makes it work?