Apple announced that the first day of iPhone 4 preorders topped 600,000 on the first day of availability, representing the largest single day of preorders ever.

Apple said in a press release that the total was “far higher than we anticipated, resulting in many order and approval system malfunctions.”

Last year, Apple sold over a million of iPhone 3GS in its first three days. At this rate, it is very likely they will sell the first million of iPhone 4 during the first 24 hours of availability.

The iPhone 4 will launch next Thursday, June 24. In addition to the AT&T network in the U.S., the phone will debut with multiple carrier partners in France, Germany, the U.K. and Japan.

Coincidentally, that’s my wedding anniversary and my wife kind of implied she wants an iPhone 4… Too bad I’m in Italy and the iPhone 4 won’t launch in time. I’ll have to think of something else… :)