If Microsoft had decided, for example, to ship IE7 with Windows 7 and release IE8 a few months later, it would have required all developers (including Microsoft itself) to test their products with two versions of IE and its components just a short time after Windows 7 was released. That, in turn, might also have given everyone another excuse to delay a move to Windows 7.
With history as our guide, it seems highly probable that we won't see IE9 until Microsoft releases its next major revamp of Windows. That blessed event would seem to be two or three years away. If Microsoft really does wait that long, Internet Explorer is in big trouble. The sneak-peak features Hachamovitch showed in the blog entry were mainly playing catch-up with the current versions of browsers like Firefox, Opera, Safari, and Chrome. Those browsers won't be standing still while IE9 lumbers its way through the development process.