My reasoning stems from the setting up of production lines. Toshiba has cleary stated many times about planning on releasing HD-DVD drives by the the end of 2005. This isn't something you set up in a short period of time. Toshiba has to have the capacity to churn drives out at this point.
When the last minute unification attempts failed, the situation forToshiba changed. They know how powerful the Sony Playstation brand is. If your Toshiba what do you do? I'm guessing that Toshiba brokered a last minute deal to make HD-DVD standard on all Xbox 360 sku's. Microsoft needs more than a couple of million drives, so Toshiba couldn't possibly launch in PC's, notebooks, and stand alone consumer players along with meeting Microsoft's needs.
The achilles heel is HDMI. I think it's safe to assume Microsoft explored the HD-DVD possibly for a long time now with Toshiba. A HDMI contigency plan I would think is in Microsoft's playbook.