AppAdvice is forecasting that iPad pre-orders will begin on February 25th in the US-only for Wifi iPads. Their source vaguely says:
According to a reliable source of ours familiar with the matter, Apple will be starting the presale of the iPad as soon as February 25th.
Apple has traditionally launched its new iPhones and iPods on Fridays, both without pre-orders recently (though AT&T has offered pre-orders on iPhones). With that in mind, March 26th has been thrown around as the actual launch date.
Norwegian Apple resellers recently had to stop taking unofficial pre-orders because of overwhelming demand.