Ok, this is going to take a few more words to explain, but overall the issue is that the forum creation workflow presents different screens depending on where you start the process.
Clean install of 3.0.6-RC2
You have a board with a category already defined, for this illustration lets call it 'Category 1' and you decide to create a new forum called 'Forum 1', so if you simply go to the ACP -> Forums tab and type 'Forum 1' in the 'Create new forum' field, select the 'Create new forum' button and leave everything on the forum settings page default, i.e. leave 'Do not copy permissions' selected and then hit 'Submit', you are prompted with the hideous new redundant "Copy permissions from" dialog that I have previously commented on (as in the problems with and why it's actually a step backwards in UI workflow) in another bug report, so far so good right? Well that is unless you choose to create the new forum a little differently.
Following this path yields different screens....
Go to the ACP -> Forums tab and select 'Category 1' and once there type 'Forum 1' in the 'Create new forum' field and select the 'Create new forum' button, you are then presented with the "old" green dialog box that prompts you to setup permissions (even though it already copied permissions by stealth), the way it actually should still be which ever menu you end up creating from.
Summary edit: If you enter a category in the ACP -> Forums page before you create a forum it automatically defaults the 'Copy permissions from' to the parent category rather than leaving it "Do not copy permissions" which should be the default rather than assuming the admin wants the category permissions to be copied by default. Never assume what an admin wants to do, leave the setting of permissions to the individual admins. I have a board with one general category and several forums inside that parent category with different permissions, copying the parent category permissions is not something I want my board to do without my explicit say so. I just do not understand this bizarre need to code behavior of the lowest common denominator users into the board, making more work for those of us who are not incompetant. :?