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According to the Wacom website:

Since the different tablet types work with different frequencies, not all pens are compatible to all tablets. Intuos supports only Intuos input devices, Intuos2 only Intuos2 devices, Intuos3 only Intuos3 devices. All UltraPad pens are compatible to PL/Cintiq (do not use UltraPen Ink or UltraPen Pencil on PL/Cintiq!), Graphire and Graphire2 devices are compatible among each other but not to other tablet types.