Siri, the iPhone 4S voice recognition assistant, is very useful! But if you've got a low-level data plan, you should be careful. Ars Technica ran some tests and found that if you use it 10-15 times a day, you're going to use about 27MB a month.
Of course the number of queries you use will be dependent on your phone usage. Some days you'll use basically none, and others you'll plow through a bunch because you're out and doing things. The problem comes from the fact that Siri sends data for every query, because the language processing happens remotely instead of just on the phone.
27MB a month isn't much, and if you don't use data a lot anyway, even if you're on the 200MB plan, you won't have to worry. But if you're already bumping up against your cap and you're also a frequent Siri user, it's something to keep in mind.
"Siri, how much data do you gobble up in a month?" Ars investigates [Ars TEchnica via Techland]