At the beginning of 2010, about 30 million people were actively using the iPhone. Apple claims to have sold 50 million total iOS devices, and that number includes both iPhones and iPod Touches. Earlier this week, 600,000 people attempted to pre-order the iPhone 4, and wrecked AT&T and Apple's systems in the process. Obviously, the iPhone 4 appeals to a few people out there. But just how many will buy it?

Morgan Stanley analyst Katy Huberty said in a report, "We see the iPhone installed base rising from approximately 30 million subscribers at the end of 2009 to over 100 million by the end of 2011. Upgrades will emerge as a key demand driver in the coming years."

Why is upgrading so important to Apple? According to Huberty, some 57% of iPhone owners won't be able to use all the features of the iOS4 because they are using older models (i.e., iPhone 3G and not the iPhone 3GS). Huberty's opinion appears to be backed up by the numbers. AT&T said that it had 13 million eligibility checks on its web site on June 15, the first day pre-orders were available. AT&T said that number is three times higher than the previous record.

So many people tried to pre-order the device, that AT&T has halted taking orders. AT&T spokesperson Seth Bloom said in an email, "iPhone 4 pre-order sales [June 15] were 10-times higher than the first day of pre-ordering for the iPhone 3G S last year. Consumers are clearly excited about iPhone 4, AT&T's more affordable data plans and our early upgrade pricing. Given this unprecedented demand and our current expectations for our iPhone 4 inventory levels when the device is available June 24, we’re suspending pre-ordering today in order to fulfill the orders we’ve already received. The availability of additional inventory will determine if we can resume taking pre-orders."

Keep in mind, the 30 million number was a worldwide figure from the beginning of 2010. Apple has been selling iPhones for nearly six months since then. It has sold millions of devices between January 1 and today. With 600,000 pre-orders taken on the first day, it is obvious that Apple is going to have a big day when the iPhone goes on sale June 24.

[Via BusinessWeek]