Please upgrade to 2.1.x here.
The 2.0 branch is no longer being updated.
I created a test user and sent my user both a private message, and wrote on their wall while having the notification preference for both on. In both cases an email was never even attempted to be sent, I'm watching my mail log and I do see other notifications being sent out of the mail queue but for some reason these are not.
Before I file a bug I assume I should gather more information on this but I don't really know where to start.