Apple can thank both its share price, which is just over $660 per share, and the number of outstanding shares available to investors for contributing to this milestone. The share price isn't staying put either, as rumors of a new iPhone and a smaller version of the iPad continue to drive the price up.
Here's the really impressive part: Apple's iPhone is bringing in more money than all of Microsoft combined. In the first quarter of 2012, Microsoft brought in $17.4 billion. The iPhone brought in $22.7 billion. That makes the iPhone alone worth more than Microsoft.
Of course, you could look at this all differently when you factor in inflation. Microsoft's 1999 total of $618.9 billion would be $851 billion in 2012 dollars. Does that change things? Even if it does, Apple is expected to cross that threshold as well.