I don't know what you mean by the statement that HD DVD (or Blu-Ray, etc.) is a totally different standard, that Encore would not be able to handle it. (First of all, how do you guys know that, are you engineers by chance?) I'm not a software programmer, but can't you just add the code necessary to make any piece of existing software do the same thing? Why couldn't there be added presets for new Encore DVD projects -- one for standard DVD, one for HD-DVD (with all its book settings, architecture, etc.), and one for Blu-Ray (with all its necessary settings)? Whether it's two pieces of software or one, why couldn't you just put the two groups of code in one software? Just like Premiere Pro and After Effects are resolution-independent -- you can import anything, at any pixel size, any screen shape, any frame rate, from any device, whatever...and output it at any resolution, frame rate, etc., even to a film recorder. Software is software, you just put whatever specs need to be put in the code. A software/hardware engineer can correct me if I'm wrong on this in relation to DVD.
That said, I do agree with John that WHEN Adobe implements high definition DVD authoring of any sort (NOT IF - it will have to to compete with the overall move to HD production/distribution), it will be later than sooner becaue Adobe tends to focus on the less expensive (relatively speaking) ways of implementing high end technology for the video production industry. (Think of the latest Production Studio package and what they've achieved.) They're not Sonic, but they're not consumerish junk either.
By the way, NEC has recently announced they will be making an internal HD-DVD burner in April (or May, I guess whenever the spec is finalized). (See http://www.macworld.com/news/2006/03/10/nechddvdr/index.php?lsrc=mwrs s)