Analyst doom and gloom predicted Apple would delay the iPad's "late March" shipping date and restrict initial sales to the US and while that now seems to be the case, John Gruber over at Daring Fireball says software and not hardware was the reason:

My sources suggest [...] It was the software, not the hardware, that took a week or two longer to finish than they’d hoped. Nothing extraordinary or unusual, just the usual hard-to-predict timing of turning software that’s almost ready to ship into software that’s ready to ship. In the grand history of major OS release date slips, one week is pretty tame.

One week for the US, http://www.imore.com/2010/03/05/ipad-wifi-april-3-preorders-march-12-international-late-april/one month internationally...