The developers responsible for the Dolphin emulator have successfully reverse-engineered the Wii U GamePad, shedding light on exactly how the piece of tech works.
The hackers found that Nintendo is using custom protocols to beam information from the pad to the console (and back), which discounts previous theories that the Wii U made use of Miracast technology.
They also discovered that the controller's firmware is upgradeable, which means that Nintendo could offer increased functionality over time. This could mean cloud-based gaming, but developer Pierre Bourdon feels its too early to say:
I don't see any reason why it could not send GamePad video/audio to the internet (both internet and GamePad might be difficult/impossible). The firmware of the chip handling communication with the GamePad (called DRH) can be upgraded, so this might be in Nintendo's future plans. Only speculation though, we didn't see anything in the firmware that would indicate they are planning to do this.
Bourdon also confirmed that support for two GamePads being used simultaneously on the same console is already present in the current firmware. Nintendo has already said that this will happen in the future, but it's nice to know that the functionality is already in place.
Below you can see video evidence of the team's work — they've successfully emulated the Wii U GamePad on a PC.