Here is a link to a free OS X app that puts Display selection back on the menu bar.
Though unfortunate as it is that your Cintiq is not working as expected with Mountain Lion, the responsibility is Wacom's, in keeping their product driver support current with Apple's operating system and hardware releases. The opportunity window exists for them to manage this customer need; providing they budget and prioritize their available resources.
The most recent Cintiq drivers were released in the last week of September.
Thanks for the free app reference Viking, I'm sure it will be very useful!
About my not working problem, unfortunatelly I don't care who is responsible between two so-called leaders companies in graphic domain. But I'm a good sheep, I'm knocking at both doors, to find some help.
Many thanks for yours anyway!