A slightly more detailed list of specs reveal the GamePad's expected battery life and that it can be charged while in use.
http://www.nintendoworldreport.com/news/30542
Nintendo has released some slightly more detailed specifications for both the Wii U and its GamePad controller.
Wii U
Height: About 1.8 Inches (not including protruding parts)
Length: About 6.8 Inches (not including protruding parts)
Depth: About 10.5 Inches (not including protruding parts)
Weight: about 1.5kg
CPU: IBM Multi-core Processor
GPU: Made by AMD, High Definition GPU based on AMD Radeon
Disk Drive: Slot loading. Runs Wii U and Wii game disks.
Controller Support: Wii U GamePad + Wii controllers. [Editors Note: Nintendo announced at their Press Conference that up to two Wii U GamePads can be used at once]
Storage: Built-in Flash Memory, SD Cards and USB storage (USB Storage unspecified).
Visual Output: 1080p, 1080i, 720p, 480p, 480i.
Audio Output: PCM Linear 6 Channel output from HDMI, along with analog output.
Internet Connectivity: Wireless Lan (IEEE802.11b/g/n), or LAN adapter option.
USB: USB 2.0 x 4 (Two front & two rear)
Wii Compatibility: Wii Software and Peripherals with some exceptions.
Wii U GamePad
Vertical: About 5.3 Inches (not including protruding parts)
Horizontal: About 10.2 Inches (not including protruding parts)
Width: About 1.6 Inches (not including protruding parts)
Weight: Roughly 500g
Screen: 6.2 Inches (16x9 aspect ratio), Touch screen.
Control Input: Touch screen, L&R Sticks, L&R Stick Buttons, D-Pad,A,B,X,Y,L,R,ZL,ZR, Plus & Minus, Accelerometer, Gyro Sensor, Geo Magnetic Sensor.
Other Inputs: Power Button, Home Button, Synchro Button, Audio Volume, TV Control Button.
Other Functions: Vibration, Inward-Facing Camera, Microphone, Sensor Bar. NFC functionality.
Sound: Stereo
Wii U GamePad Stylus
Battery: Lasts 3-5 hours depending on brightness and other situations. Recharge time is 2.5 hours, can recharge while being used.