Hi,
I've got the FAN Discount applied to my iPhone (G1, 16gb) on a Family 550 plan. I believe if you want to keep the FAN Discount, then the iPhone cannot be on the primary line. I switched my # to the additional line and switched my wife's line (non-iPhone) to the primary line. I still get my FAN Discount and my monthly bill comes out to $75 total!
If you have a FAN discount and you only have 1 line, then I think you are out of luck. But, I could be wrong.