Glad this info could be of use to others. I've just got a quick followup regarding the root-cause.
The problem returned as soon as I added my Macbook back as a managed device into a Mountain Lion Server Profile Manager -- deployed the payloads and rebooted.. (Reboot is important here for those using management profiles).
To be clear, I do not deploy the 'Parental Control' payload, it's left completely disabled, but despite this the Macbook still enabled Parental Controls for my admin account.
I'm sure Apple will fix this in an upcoming update - let's see.. 🙂
Also, and like I previously mentioned, the default in Parental Controls is to disable Dictation. This seems an odd choice to me seeing as all other defaults are not restrictive in nature. Maybe this is not intentional too.
Apple should also update Parental Control preferences in Profile Manager. Currently there is no way to override Dictation settings (and also some new Game Centre related defaults).
Phew -- Glad I got that off my chest. Really enjoying Mountain Lion otherwise. A nice update really polishing Lion.
Paul