Because you then go three months without a machine or, as in the case of my daughter, limping along on a windows 98 machine that is incapable of doing what's needed, when the option is to get a modern, fast Vista-based machine that will require me to throw in a DVD and do the W7 update in a few months.

Yes, there's the risk that a Vista>W7 machine won't fare as well as a native W7 machine, or the upgrade process will be flawed. But the other option means she spends 2 months in school barely being able to use a computer.

If you're buying a new machine just to have one then yes, waiting makes sense.