I used the unlimited messaging plan cost for an individual by mistake.
Unlimited messaging with a family plan shared between all lines is $30/mo.
For the person I replied to, it would cost an additional $5/mo for all 3 iPhones to have unlimited messaging.
Don't get me started on who can MMS and who CAN'T!!! ;^)
The iPhones - at least a 3G or 3GS will have MMS access this summer - something I haven't missed in the least, but I understand it is in end all for many.