Worldwide, I would guess 20-30 million tops
A large part of the iPad market already has one or more, and with 15 million already in circulation even those who sell the iPad and buy a new one, it means that the person who bought the used iPad won't need a new one for a while ( one less iPad 2 customer)
They own A HUGE share of the tablet market, and that is unlikely to change, but I don't see more than 15 mil for the us market, and 15 million for overseas markets.
I suspect the largest increases will be out of the USA as there will be a larger "fresh" market to buy the iPad 2. If the iPad was a replacement for a pc or laptop or phone, I could see it being larger in sales, but the iPad2 can't do the job a pc can... People will still need the laptop/pc, and so the iPad will still be a luxury item instead of a need per se.