Thanks to many iPad 3G service providers offering month-to-month plans for 3G cellular data connectivity on the iPad, many WiFi+3G iPad owners either don't use 3G connectivity at all times, or never use it. That, however, won't stop the iPad from incessantly asking if you'd like to sign up for a cellular data account every time you're out of range of WiFi. Hitting "Later" 1,000 times won't change the frequency either. It seems the iPad can't take a hint.
To stop your iPad from badgering you about signing up for a 3G data plan, simply go to
Settings -> Cellular Data
and set cellular data to "OFF".
Even if you don't mind the constant questioning, you should turn 3G/cellular data off on your iPad if you don't intend to use it. You'll save battery life.