Apple appears to have sold out of its initial US inventory. Instead of quoting a delivery date of March 16, all models now show ship dates of March 19. Even though the shipping date has slipped, if you order online now, that means you can still have an iPad within about 7 days of its official launch date without having to wait in line.
If you are brave and having nothing else to do, you could wait in line at an Apple retail store and HOPE that by the time you get to the front of the line the model you want will still be available.
If you are considering getting the device, do so now before the ship date gets further out.
While waiting for your device, you may consider getting some accessories before those items are sold too!