$message passed to hook implementations is not an array.

E.g. hook_privatemsg_message_insert() receives arg(0) ("$message") as an object.

I would have added a patch for privatemsg.api.php, but a little code-crawling through the module's folders reveals that this goes throughout a lot of doxygen comment sections as well. Seems like you suddenly realized it needed to be changed from array to object but changing documentation got lost at some point ;)

Probably related to at least #1127246: Sending new message PHP fatal error (Cannot use object of type stdClass as array).

Someone[tm] should look through all the code somewhen[tm]. ;)

Edit: Looks like there's really some documentational update lack. $message['author'] has not just turned into $message->author but is now also no longer a UID but a user object...