The fine print on that page says:
"Coupon codes are not valid anywhere but the HP online store"
"Coupon codes are not valid anywhere but the HP online store"
Remember, I'm dumb. I mean, I write PHP, fergawdsakes. I'm going to have to be led by the hand here.
I went to their web store http://www.shopping.hp.com and searched for "touchpad" but all I found for sale were accessories: cases, stands, cables, etc.
Maybe this coupon signup is some kind of marketing survey, to help HP decide what to do with the touchpad.
WebOS is a hell of a platform; it's a damn shame it's probably going to die on the vine.