Pre-orders for the new Apple (Nasdaq: AAPL) iPhone 4 have been so strong that, unable to keep up with demand, AT&T (NYSE: T) had to halt sales last week. On its own website, Apple itself pushed back the date new owners can expect to receive their pre-ordered phone. Even though the phone will be in stores June 24 "in limited quantities," it won't ship from Apple until July 14.

So what's causing the avalanche of iPhone envy? Could it be the high-def video, the two cameras, retina display, or multitasking? Well, that's only part of it, according to an article in Bloomberg Businessweek. A timely and very smart promotion by AT&T "allowing any iPhone customer eligible for an upgrade in 2010 to buy the new phone at the lowest price" was a major catalyst. That means a major portion of the pre-release frenzy is being fueled by current owners of older models.

Also good for AT&T, says the article: All of these upgraders will be locked in for two more years, during which Verizon Wireless (NYSE: VZ) presumably will become the second carrier for the iPhone.

I still have a first-generation iPhone, and I'm itching to upgrade. The high-def video and vastly improved camera are major selling points for me. If you're planning on getting the iPhone 4 soon, let me know below what's drawing you in -- along with whether you'll be upgrading or if you're a first-time owner.

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Fool analyst Rex Moore is willing to wait a few weeks to upgrade, after all the kinks are worked out. He doesn't own any companies mentioned in this article. Apple is a Motley Fool Stock Advisor recommendation. The Fool has a disclosure policy.