How would that work for family plans? Add $9.99?
My family plan on the 2G iPhone (1) is $49.99 for 450 min / 2 lines plus $20 data ($69.99 plus tax and fees). This is an unadvertised plan, but it should be available if you ask nicely; I think the 700 min plan is another $20.
EDIT: Of course, the 3G (and presumably 3GS) data plan is $30, not $20 as above.