I travel all over the world. The thing you need to ensure, is "data roaming" is turned OFF. This is OFF by default. As long as that is the case, you CANNOT be charged astronomical data (or any for that matter) usage.
*AT&T Customers* With your iPhone setup exactly like it is now. You'll be able to receive text messages for free (incoming text messages go against your current plan...so if you have unlimited texting...you'll get unlimited incoming texts). However, outgoing text messages will be $.50 a piece.
Also, you'll be able to receive phone calls (don't answer them, or it will cost you approx $4.00/minute). However, this is useful in the case of an emergency.
I like being able to receive all my text messages and phone calls while abroad. I won't respond to them (or answer the calls) unless absolutely necessary. What I do, is wait until I'm on WiFi and either email the individual back and instruct them to email me while I'm overseas, or I'll use iMessage if the other party is an iOS user too.
Again, just ensure data roaming is turned OFF (this is the default setting), and you'll be fine.