Currently Being ModeratedJan 14, 2013 1:31 PM (in response to meggomyeggo)
Yes and no. You will not be charged for an SMS/MMS message, but it does use cellular data. As long as you don't overrun your monthly allowance, it won't cost you anything extra. It does incur data use, though, and data is not "free".
Currently Being ModeratedJan 14, 2013 1:35 PM (in response to meggomyeggo)
iMessage sends data. If you are not in wifi these data will be transferred over 3G/4G network.
From few bytes for a simple text to many MB for a video.
So it's up to your data plan program.