Completely forgot about this tweak. Another reason to jailbreak
It would be a nice feature if Apple had a sub-setting which allowed users who have unlimited data to preset a toggle which would disable data usage warnings throughout iOS.
...Or if iOS automatically communicated with your wireless carrier and adjusted itself accordingly.
Heck, if they got visual voicemail to work properly, and years ago at that, then I'm sure it would be quite possible to issue a simple true/false query to the network and make the result visible to iOS so that it could behave accordingly.
However, I'm inclined to believe that Apple will always err on the side of caution and warn you about potential data charges, even if the the technology to avoid the warning (when unnecessary) is available. This helps Apple avoid a scenario where hundreds or thousands of pissed off customers who get charged a fee for exceeding a data cap cry foul and attempt to band together and start a class action against it, against the carrier, or against both.