It can be taken away from the system a good distance, but not overly far.
http://www.nintendoworldreport.com/news/31799
The Wii U GamePad has the best performance when within approximately 24 feet of the system, according to Bill Trinen in an interview with 1UP.
"They had a very precise number at one point. In meters it's like eight meters. In feet it's about 24 feet. That's for best performance," Trinen said when asked whether there was a limit to the controller's range.
Under good conditions, you will likely be able to push the system's wireless capabilities a little more, or take a dip in performance if you would like to sit a few extra feet away.
Wiimote was said to be 10 feet, yet there's proof that it's range is actually around 400 feet. Then again the GamePad has to receive a live video feed of the game instead of just relaying motion and button input...but we'll see for ourselves what the range is once we've got our hands on it.