If you’re early enough beating the AT&T meltdown, you might already have a confirmation for the iPhone 4 pre-order. I really want the White iPhone 4, but for now I reserved one at the Apple Store. After numerous tries, I succesfully reserved a 32GB Black iPhone 4. For some reasons, whenever I tried reserving the 16GB version, it kept failing.
UPDATE:
Apple has just disabled iPhone 4 Reservation through Apple Store App, citing high demand.
You might have easier time reserving iPhone 4 using Apple Store App for the iPhone.
