Each time you use Siri, your phone will be connecting to the internet both through your phone network or Wi-Fi, this activity of course eating your data.
And the cost you have to pay is increasing fast as you are connecting to the internet too often.
And Engadget has listed a comparison data usage between A5 based device with its older A4 brother, and here is the result:
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 you can see the A5 device consumes almost twice data than the older models. And currently there are millions of iPhone 4s users. This situation makes two giant cellular telephone plans companies, Verizon and AT&T, having a hard time in their network because of Siri alone. The effect of this AT&T has to deal with dropped calls, slowdowns of their data service.
So what is the solution for the problem to deal with this little bandwidth appetite? In my head now is unlimited data bundle. That's it!