I can understand why Microsoft isn't interested in making a BRD add-on (only 3% attach rate for HD DVD), but they're basically handing over sales to Sony. Here's why:
As a movie buff, I'm going to get a BRD player now that the format war is over. As a 360 fan, I'd love to stay connected to Live while using an add-on BRD drive that's cheaper than a stand-alone player. But if they don't release one, I've got two options - buy a stand-alone BRD player or a PS3.
Now, as a gamer, if I have to buy a stand-alone player, why would I not want to just buy a PS3? I'm not a fanboy, I realize that the PS3 has great features, but it was just too expensive to justify the cost. But now that it's dropping in price and BluRay is the victor, I have fewer reasons not to buy one.
Sorry, this is lengthy but one more point - doesn't Microsoft realize that the low 3% attach rate for the HD DVD drive was because people were holding off until the war was over? Now that BluRay is the winner, it's obvious they would have a much higher attach rate with a BRD add-on.