Just a question about the data plans. Since this is a non-contractual data plan, pre-pay in nature, must A T & T be my cell phone provider, or is this something not associated necessarily with my cell phone? In other words, could Sprint be my cell phone provider, but A T & T my iPad data provider? Thanks in advance for your responses!!!

Yes the iPad is a separate Data plan from your existing cell phone account.

The iPad will incorporate AT&T's Micro Sim and will allow the iPad user to purchase monthly service (pre-Pay) for the data plan they wish to use monthly.

If you want to use one month of data pre pay plan and nothing the next, that is possible as well as one pre pay package one month and another the next.

The plans are activated on the iPad, as Steve Jobs has already indicated during the announcement Keynote of the iPad.
So you chose what data plan is needed by you during I'm assuming 30 day intervals.
Otherwise I can see people changing their minds constantly and AT&T having to pro rate days in a month for proper charges. Otherwise to simplify I would imagine they just use fixed payments and base them on the 30 day interval periods.

The two plans announced by Apple through AT&T are;

1. $19.95 for 250M of Data (in a month)
2. $29.95 for Unlimited Data (in a month)