There have been many threads on this the past couple of days yet I am still able to add the Unlimited plan to my iPad and I have not signed up for a data plan yet. So while the website might not have the plan if a person is just now getting a 3G iPad they still have the ability to sign up for the plan.
Since its not being advertised either on the Apple or AT&T websites its only a matter of time before it will no longer be an option. I'd give it a month--save some type of epic reversal on AT&T's part. This move by AT&T just makes competing carriers more attractive to me. Data doesn't have to be built in -- just get a smartphone with tethering/hotspot capability.