This is true. I got a rep to allow me to upgrade to a 3GS at $199 even though I wasn't elligible for the $199 price (bought a 3G 10 months earlier). Every rule can be broken, you just need to find someone to do it.
Only if the rule is just that, a rule and not a system limitation.
Here's what I think happened, to get the iPhone data plan, they are looking at the former IMEI # on the account since these users were past iPhone customers. So some reps take the time to look it up while others don't. But if you never had an iPHone, I'm still saying you can't add the iPhone data plan.
But there has been a posting on Gizmodo stating that any unlimited smartphone plan will work.