Apple’s iPhone gravy train keeps rolling on, but it’s having trouble keeping up with initial demand, as shipping estimates for pre-orders the latest version have slid to 3-4 weeks, as noticed by MacRumors.

When pre-orders for the iPhone 5 opened up last Friday, Apple quickly sold a record-breaking 2 million units. Within about an hour, shipping time quotes quickly jumped up to 2-3 weeks. Those estimates held steady for a few days, until they increased again on Tuesday evening (US time).

Anticipation for the newest iPhone continues to ripen ahead of Friday’s release. Early reviews for the hot pre-selling gadget are already out, and most of them are glowing. Apps have also started to get the full iOS 6/iPhone 5 treatment.

The iPhone 5 has a taller 4-inch screen, 4G LTE data support and an A6 processor that’s billed as being twice as fast as its predecessor.

Image credit: Justin Sullivan/Getty Images