The wi-fi only models will be available on this date with the 3G versions being released later in the month.
The Apple website states that preorders will be taken from 12th March via the Apple online store. You can arrange to have one delivered or reserve one to pick from Apple retail stores.
Outside the USA
Apple has also announced that the iPad will be available in both Wi-Fi and Wi-Fi + 3G models in late April in Australia, Canada, France, Germany, Italy, Japan, Spain, Switzerland and the UK. International pricing will be announced in April. Additional countries will follow later this year.
This is a month later than was originally announced by Apple for international launches. The original date quoted was late March for the wi-fi only version. Country-specific iPad pages are yet to update to reflect this news. The UK iPad page, for example, still says “Wi-Fi models shipping in late March. 3G models shipping in April. UK pricing to be announced.”