My guess is that the way to promote Linux is to give people a safe, secure and no-fuss way to try out different distros before committing to one.
Assuming that the average Linux distro is about 600-700MB you could get about five onto a DVD. That's cool but given the number of distros out there you'd only get a small taster of what's on offer (and it would only include the most popular ones ... Ubuntu, Kubuntu, PCLinux ...).
But Blu-ray and HD-DVD could change all this. Assuming that the price of discs falls to a point where it's sensible (say the cost of a DVD) then you could squeeze a good selection of Linux distros onto a single disc and make a monster Live disc. In the past few weeks I've downloaded a handful of distros and having them on one disc sure would have made the process of wading through all the different distros a lot quicker and less hassle.
Finally, a use for HD-DVD and Blu-ray! ;-)