iPhone 4S users are consuming around "twice as much data as the previous iPhone model, and even more than iPad tablets due to increasing use of online services like the virtual personal assistant Siri."
Of course, this relates only to those sampled, but there's no question that the inclusion of Siri gives users one more reason to tap into their data plan. According to prior studies, the iPhone 4 used only 1.6x more data than the iPhone 3G, while the iPad 2 consumed 2.5x more data than the iPhone 3G. As time marches on, more and more websites are tailoring themselves for mobile use, and there's obviously no stopping the proliferation of data-sucking apps. Whatever the case, we'd just encourage you to mind that data tier (if you're in one) -- wouldn't want to have to ask Siri how to raise money to cover overages, no would you?