The simple answer is yes. iMessage uses data from a data plan to send messages over the 3G connection and in turn does not use any text message costs or allowances. You will not be able to send and receive iMessages without WiFi if you do not have a data plan...or another way...if you do not have a data plan, you will need WiFi to send and receive iMessages much like the iPod Touch.
I hope this helps!
The really exciting thing about iMessage is that it doesn't use the SMS functionality of the cellular network, so all of a sudden I'm getting more milege out of my 200 message/$5 per month messaging plan. The last thing I want is to pay $15 bucks a month to AT&T for unlimited messaging, and my heaviest texter is a friend who has a 4 updated to 5.0, and now she can text me unlimited without killing my paultry 200 message plan. Typically, besides her, I send less than 30 a month.