I don't know just how much of a vested interest Microsoft has with HD-DVD, but to go in with Toshiba and pay Warner money for 300 on HD-DVD over Blu-ray would be a play to win move. A next step further move would be to make 300 an HD-DVD release only by also leaving out the DVD format. At the time of the release I would have a $400.00 or $300.00 player ready to go.
Things to consider:
Microsoft has money.
Toshiba is not in the hole like Sony is, and has been.
HD-DVD and Blu-ray are next-gen formats Just like the Xbox 360 and the PS3 are, thus making it a next-gen release. (Halo 3 360 only no xbox)( Metal Gear Solid 4 PS3 only no PS2)
Sony for a while had paid Take Two for the Grand Theft Auto series, and EA has paid the NFL for NFL only football games.
I know HD-DVD only may be to hard to do, but DVD and HD-DVD only would be I think very possible.