"Over the course of two weeks in June, Apple will deliver more new phones than any mobile handset manufacturer in history." So says an article by Tom Yager.
To understand why this is so, read the article. Hint: the free software upgrade available to all iPhone users represents a huge improvement, and the new iPhone 3GS is a compelling upgrade.
Full disclosure: I am long AAPL and I own an iPhone 3G at the time of this writing.