Also, the reason there are two tiers (250 and 'Unlimited') is that many/some/whatever people either do or don't (look at the argument either way you like) spend a lot of time in WiFi, and don't need a lot of 3G data.
1. I have WiFi in all buildings at work, and WiFi'd FiOS at home. So, I have a WiFi iPad that I leave at work so I don't need to carry it back and forth. (Excessive? Apple stock has been very good to me...
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).
2. I also have a 3G iPad for use at home and on vacations and outside. I'm starting with the 250 to see how often I'm using 3G at all. But I know on my iPhone I almost never use 3G except in a store googling something, etc.
Now, on a similar subject, which probably needs a new thread, I'm wondering how bad your WiFi connection needs to get before the iPad flips to your 3G. I'm pretty in my first testing last night that my 3G usage is higher that can accounted for by the 'pure 3G' testing I did. (I.e., with WiFi off.)
I think it's failing over to 3G when it thinks the WiFi is either not there or 'poor enough'. This is important, because people with 250 MB may be frittering it away when they don't know they are.
Right now I have the 3G off on that iPad when inside, but that's a pain to have to turn it on and off all the time. The point is that it should be automatic, which means if you eat 3G when WiFi isn't good enough, more people will be forced to purchase unlimited perhaps.
I don't know of any way to tell which 'band' you are actually using at any given time, if both are turned on. The WiFi indicator may not actually move, and the 3G doesn't anyway. Anyone?