Although conspiracy theories are fun, note that Micrsoft has a major stake in HD DVD and only now when they are clearly losing this war are claiming that both HD DVD and Blu-Ray would soon become obsolete technology. Making such claims wouldn't make much sense from the perspective of a faked format war.
What major stake? Did they do anything other then allow Toshiba to make an HD-DVD drive for the 360? I wouldn't call that a major stake.
And calling it a conspiracy theory does not prove it didn't happen, especially since there is precedence for companies to keep secrets, such as nearly every third party developer keeping mum on the Wiimote's design until Nintendo revealed it. So it isn't unreasonable to assume that with ALMOST EVERY electronics company pushing HD (even Nintendo made the Wii with some HDTV standards), a couple companies, that had worked together before, would try a simple publicity stunt to get attention to their formats.
And it is simple. Most conspiracy theories involve elaborate plans. This is simply that Sony and Toshiba agreed to make two formats, and keep them around as long as it took to get people talking, even if one sold better and had greater support. It's just a larger scale game of good cop, bad cop. That's hardly a conspiracy.