Bottom line: no one knows when iPhone will ship, and we can't be sure if this rumor is true, or if components in April mean Apple is on target, early, or late.
But I'll say this. The iPhone will need more testing than any other Apple product. At MWSF Steve showed how integrated the phone would be with Google and Yahoo, and that there were considerable accomodations being made by Cingular for such things as visual voicemail.
And the hardware is brand spankin' new. Multitouch, proximity sensors, and because it's a phone, it has to "just work."
Apple and Cingular will have to roll out a few thousand of these by May to distribute to those special Cingular and Apple employees willing to trade their first-born and an air-tight NDA signature for the hippest toy under the sun.
Perhaps these April shipments are for a sampling quantity to refine the manufacturing and get test units into the field.