This is happening to me. Just bought a WiiU a few days ago.
Sharp Aquos LC-42D65U
It's totally an issue with the WiiU because my 360, PS3, Laptop, and Desktop PC all work just fine at 1080p using the same HDMI input, and same HDMI cable.
This is very disappointing because this TV is absolutely perfect for all my other gaming needs.
I've seen people say to set the WiiU to 1080i or 720p, but that's absurd. I bought a 1080p TV to use as a high-definition monitor for high-definition content. The WiiU is capable of 1080p output, it should be able to output a properly compliant signal regardless of what display it is connected to.
What do I do at this point? Exchange it at the store for another one? Only to sit through hours of updates and configuration and have it happen all over again?
I can't imagine it's necessarily a hardware defect, so do I have no other recourse than waiting for a new firmware patch that [hopefully] solves the issue? My TV's firmware is as current as it can be, so I'm not left with too many other choices.
Xbox One is looking even better now, especially with that $50 price cut.