As noted on 9 to 5 Mac, Apple traditionally begins selling new products like iPhones or iPods on Fridays. The closest Friday that is approximately 60 days after the iPad announcement on January 27th is March 26th, 2010. If Apple decides to take pre-sale orders of the device online for express delivery on March 26th, they would probably do this about a month in advance in order to better judge the demand for the product.
The iPad must pass muster with the FCC before it can be offered for sale in the U.S., so if this rumor is true we'd expect to see the device approval sometime in the next week. The 3G model, which will be offered about a month after the Wi-Fi version, will require separate approval due to the inclusion of a cellular modem in the device.
[via 9 to 5 Mac]