Neven Mrgan posted a nice graphic and writeup on his blog that demonstrates the potential for iMessage to disrupt the normal quantity of text messages iPhone owners use. I too had the 1000 for $10 plan and so did my wife so I was contemplating turning off our text plans. Then, as if on cue, AT&T came along with this and it and now doesn't make sense to cancel my text plan- in fact it makes more sense to bump up to the unlimited plan which also makes your mobile-to- other network mobile calls free. That allowed me to drop our plan to half the minutes as we hardly ever call anyone on land lines.
I hate text messaging charges, but it is hard to complain with a bill that is $20 cheaper each month.