When Apple first announced the iPad, they said that the Wi-Fi models would be shipping in late March. While no one has heard any official news from Apple, the date flying around is March 26th at 6 PM.
According to the Examiner, this date makes sense because Apple employees are reportedly getting trained for the iPad beginning on March 10th. The post also says that TV ads will begin Mach 15th. Finally, those who camp out to get an iPad on its launch date will receive a "special gift."
The 3G model iPads are scheduled to be released sometime in April.

[...] might be due to Apple wanting to make sure everything is perfect for the iPad launch, which is rumored to be happening March 26th. Or, it might be an update for the iPhone OS 3.2 beta 3 SDK, which has [...]
[...] report that showed up earlier this week said that the iPad would be on sale on March 26th, turned out to [...]