I highly doubt you will need to purchase a sim. I would imagine it will come with the correct sim for the country you bought it in.
Nope, Steve said that they would be sold unlocked so any 3G iPod would be likely to work wherever telcos were selling micro-SIM cards. Then later in the year there would likely be even better options for those who bought a 3G iPad in the US and took it home as Apple and the telcos work out their own specific deals.
It will work like most pre-paid iPhones down here. You buy the iPad, grab a micro-SIM, activate on the iPad itself and you're done.
Pre-paid data credit down here normally lasts 30 days with some more expensive options lasting 12 months. I get 4GB for a A$49 recharge that lasts me 30 days. I can get 12GB for $149 but that lasts a whole year.