I'm really impressed with the way they are handling 3G data on the iPad. You really can't go wrong:
1) I will use WIFI whenever I'm at home or in free hot spots.
2) I will start with the 250MB/mo plan at $15 and see how it goes the first month. Thankfully, they have implemented a data usage meter so you can keep track of how much you use.
3) Furthermore, you can upgrade at any time so if you use more than normal one month, just upgrade to the Unlimited plan until your usage goes back down. There's no contract and you can change plans, so you have nothing to lose!
I imagine 250MB will be enough for me most of the time, but I will certainly upgrade to Unlimited for those times when I need more (trips, etc). It really all depends on how much I use this thing outside of my home, and I won't know that until I actually get to use it.
The only question I have is what happens if you get to 250MB without checking the data meter? Will data access stop working automatically? Will you automatically be charged extra if you go over 250MB? Will it charge you by the megabyte after you go over, or will it upgrade you automatically to the unlimited plan for that month?
I'd prefer to have the option, but I'd rather it just cut off my data access if I reach the limit and give me a message. I don't want to always have to worry about checking my data meter to prevent overcharges.