The good news is if you do accidentally trash your GamePad, you're not up the digital creek without a touchscreen.
Gimme Gimme Games contacted Nintendo customer service about getting a replacement if something goes wrong, and got this email back:
"To address your inquiries directly, I can tell you that individual replacement Wii U GamePad controllers will be available from Nintendo Consumer Service for a fee. In regard to warranty coverage for that accessory, at this time no specific announcements have been made. However, it’s worth noting that Nintendo warranties do not cover physical damage."
In other words, chuck the GamePad like a frisbee and replacing it is on you. Nintendo hasn't communicated how much this "fee" will be, but it'll likely be between $125 and $150 if the Japanese pricing is any indication.