I think if you pay 30 for data it should work on any device you own, as long as you are using one device at a time. I would buy an ipad and iphone 4 if this is the case. You really should be able to buy one 3g data plan for your iphone/ipod/ipad, just make it so you can only use 3g on one device at a time.
But I could see having an iphone and swapping cards with the ipad, being able to swap an ipad sim to an iphone or iphone sim to ipad would make sense. Most people will not pay $60 for 2 data plans, and in reality they should not be afraid of people swapping sims, it's not going to be huge deal because at home you got wi-fi, but sometimes you might need 3g on your ipad and swapping would be a bonus.
*(have no Apple products yet, Linux, Win7 and Android user, but got my parents an ipad and was pretty impressed, it's certainly a great entertainment platform, and the parents love it, easy to use and does everything they need)
But I could see having an iphone and swapping cards with the ipad, being able to swap an ipad sim to an iphone or iphone sim to ipad would make sense. Most people will not pay $60 for 2 data plans, and in reality they should not be afraid of people swapping sims, it's not going to be huge deal because at home you got wi-fi, but sometimes you might need 3g on your ipad and swapping would be a bonus.
*(have no Apple products yet, Linux, Win7 and Android user, but got my parents an ipad and was pretty impressed, it's certainly a great entertainment platform, and the parents love it, easy to use and does everything they need)