There were some recent rumors that XBox 360 has some plans to offer a Blu-Ray external drive. However, I just read that the XBox team announced that they are fully commited to HD-DVD and have absolutely no backup plans for Blu-Ray.
The question of whether Xbox 360 has the flexibility to adapt to consumers’ needs is a different issue entirely. Xbox 360 is a future-proofed system – one that allows us to add features as consumers demand them - as evidenced by our offering of the HD-DVD drive as an accessory. Current reports indicating that we have a back-up plan, which includes Blu-Ray support are incorrect. At this point, we’re fully committed to HD-DVD and have absolutely no plans to support other optical formats."