Watch Dogs for Wii U has not been cancelled. The rumors of GameStop calling customers and notifying them of the cancellation of Watch Dogs for Wii U may have been true, but it was all due to an error in the retailer's system.
We had some initial reservations regarding the cancellation of the game, following Wiitalia's report, so this doesn't really come as a surprise. Kotaku may have their "reliable" source, but all it took from us was just a simple phone call to our local GameStop retailer. Journalism — or the simple act of calling GameStop and asking if I could place a pre-order for the Wii U version of Watch Dogs (I could).
So why all the rumors? Turns out GameStop Italy experienced a glitch in the system that caused the Wii U version of Watch Dogs to become de-listed. As you can guess, rumors spread like wild fire. The Italian arm of GameStop has confirmed "tomorrow the reservation[s] of Watch Dogs will restart."
As for those of us in the United States, I don't believe they ever really stopped taked reservations. Sometimes it's as simple as checking with the actual retailer, folks.