But, what are the risks for a developer when reverse engeneering a camera's firmware?
First and foremost, you risk bricking your camera.
The following comes with the very standard "I am not a lawyer" disclaimer.
As far as legal risks go you shouldn't have to worry about the DMCA, that only refers to copyright protection circumvention. Because there are no copyright protection mechanisms included on the camera, you shouldn't be afraid to reverse engineer anything. Check out:
http://www.lawtechjournal.com/articles/2004/03_040907_hecht.php and especially read the paragraph starting with "Defining the limits of acceptable reverse engineering."
You are not restricted by the DMCA, however, you are still bound by copyright law. Sony's code (in your case, basically anything not released via the GPL) is copyrighted, and you are not allowed to post it in any form, assembled, disassembled etc. For example, we don't host any firmware dumps of any cameras because they are copyrighted by Canon. We can, however, post instructions on how to dump, the firmware, and how to interpret and look at it.
On a side note, how similar/different is developing for canon and for sony?
They are very different. Canon devices run on an embedded OS, either DryOS or VxWorks, and it looks like the Sony devices run on some form of Linux. Probably the only similarity is they probably both use an ARM processor.