I have also got this problem. I have upgraded a 1.4.4 board to final release this afternoon....everything went very smoothly I must add.
The old board had one rank setup, which was correctly upgraded and assigned to the admin user as it should.
I can add further new ranks into phpbb 2.0 admin without any problems, and they add themselves into the list to edit in the future.
However the problem I have is that I can't assign any of these new ranks to users. I can lookup a user, and sure enough the ranks show in the drop down list....however if I select a rank, click submit they don't actually take effect.....do a search on the same user again shows the message 'no special ranks assigned'. However do a search on the orginal admin user, sure enough the selected rank is listed as you would expect.
So coding buddies what you broke