Apple started taking pre-orders for the iPhone 4 yesterday, and as it turns out, the company’s approval system broke down under the strain of a whopping 600,000 pre-orders. Give us a shout if you are one of the lucky ones.

Apple sent out a press release to explain the situation, boasting in part, “It was the largest number of pre-orders Apple has ever taken in a single day and was far higher than we anticipated.” AT&T said in a separate announcement, “iPhone 4 pre-order sales yesterday were 10-times higher than the first day of pre-ordering for the iPhone 3G S last year.”

Because of yesterday’s hi-traffic and “current expectations for . . . iPhone 4 inventory levels,” AT&T has opted to suspend pre-orders until the iPhone 4 becomes available on June 24. The carrier suggests it wouldn’t be able to fulfill existing orders otherwise. Pre-orders might resume if AT&T manages to secure additional inventory, though.

To put this in perspective for you, In the first quarter of last year, when all iPhones still sold for $199 and up (just like the iPhone 4), Apple managed to ship 3.79 million of the devices. Last quarter, shipments of all iPhones $99 and up totaled 8.75 million. Selling 600,000 $199-299 devices in one day certainly bodes well for the Mac maker.