The Gartner and The Burton Group, two highly respected groups of analysts, confirm what consumers have been saying for the past year or two: June 30 2008 is simply too early to stop printing new licenses of Windows XP. The analysts had this to say about any OS transition:

A good rule of thumb in any OS transition is that you have to have the original and new products available for at least two years to handle customer (migration) needs

Analysts over at Gartner explain why, exactly, Microsoft should be easing off the "upgrade your OS" button. While Microsoft does a fine job of ensuring as much application compatibility as possible, they simply cannot patch up every single application on the face of the planet, especially "home-brew" applications and applications from minor things. Pushing the cut-off date off by seven more months (so that the cut-off date would be two years after Vista was first released) would give everyone plenty of time for transitions.

There are a lot more reasons, which are just about all listed at the source link, if you're interested.