Siri—the equally impressive and gimmicky iPhone 4S' smart assistant—connects to Apple's servers every time you ask for something or dictate a message. It sends your query and analyses it, returning you whatever it thinks is the correct answer or action.
This means that, depending on how much you use it, it may affect your data plan. But how much?
Jacqui Cheng at Ars Technica tested the consumption using various questions. She found out that, on average, you can expect Siri to consume 63KB per query. According to her calculations, that would put about 4 to 30MB on top of your usual data consumption, depending on your usage.
Hardly dangerous for any plan, but a useful tidbit for all those three iPhone users who spend all their day trying to have virtual sex with Siri. [Ars Technica]