That will make present Xbox360 owners real happy--having them pay for a entire new console. If they, have a internal replaceable unit that users can install on current consoles, that would be one thing, but even at this, your looking at potential voided warranties, accidental damages, frequent returns, etc. as you have users cracking open their 360 to replace their drive. I'm going to go out on a limb and say I don't think this would be a great strategy at all--cost wise or consumer-friendly wise.
Furthermore, if it is an internal or external add-on, they better be making it capable of more than 720P output from what is expected. Also, as we've discussed add-nauseum is the fact of how external add-on drives historically fail, and I don't see this drive changing that history. Also, they better find a way too sell it less than the expected and at one time temporarily posted price of $199 which would put them at the same exact price of a built-in Blu-ray drive PS3.
Also, from what I understand is that the add-on for the Xbox 360 as it is currently, will only be capable of playing HD DVD movies and not capable of playing games on the HD DVD format. Still want to scuttle your plans for a PS3? Probably, as we all know you are a fanboy of everything Microsoft, from their backed HD format of HD DVD, to their interactive layer in iHD, to their encoding software in VC-1, and now of course to their gaming console in the 360 despite having a standard DVD drive, and only 1080i output.
So, honestly, scuttle away as I wouldn't expect anything less from a Microsoftie, but your considerations are just that, yours. Let others consider the facts, and practice their free agency upon those facts as to what they'll purchase, and not on fear mongering from you.