Yesterday was the 1st pre-order day for iPhone 4 in the U.S. and a few other countries (not Canada):

Yesterday Apple and its carrier partners took pre-orders for more than 600,000 of Apple's new iPhone 4. It was the largest number of pre-orders Apple has ever taken in a single day and was far higher than we anticipated, resulting in many order and approval system malfunctions. Many customers were turned away or abandoned the process in frustration. We apologize to everyone who encountered difficulties, and hope that they will try again or visit an Apple or carrier store once the iPhone 4 is in stock.

Press Release

Very nice of Apple to not throw AT&T under the bus as I'd put money on the issue being AT&T's system, not Apple's.  Either way, 600,000 iPhone 4 pre-orders in one day is a very large number.

Good news for Apple, but disappointing for us in Canada as I wouldn't hold out hope for an actual July ship date in the great white north.

Via Macrumors.com