Maybe you weren't saving your adjustments to the DNG but that has always worked. I was mostly meaning sending files to other computers or other people, but all using Adobe software, and didn't talk about cross program processing. The adjustment parameters are unique to apps, but Adobe's parameters are visible - so someone could conceivably try to translate them (eg translating WB from Adobe's measures to Aperture's). It would be a lot of effort and would never be perfect, but might save time, though I doubt it..
re Aperture, no doubt those Apple guys gave you all sorts of convincing reasons why it wasn't ever going to possible and that if you were truly faithful to Steve Jobs you would believe reading DNGs as DNGs was a really bad thing. Then things changed with 2.1 (might be 2.0).
yeah, I was very carefully doing that, taking raw files, processing them, saving as DNG. Phase, notably, if memory serves, I'll have to check my notes, or just do it again, but the point of the test was to address some users wanting to use the "handles" in some processing, like Raw Processor or Phase, and then working with Adobe and getting worked files rather than back to the starting point. That would be one of the points of working a DNG file, I'd think, the ability to save your work across processing "platforms".
As far as Apple goes, ...actually I had a really interesting talk with one of the not so senior developers at Apple who was in on my Aperture training class. (they like to send the engineers to a class so they get both real-world experience as well as a complete view of the package, something a lowly line engineer doesn't get apparently). He was telling me horror stories about how impossible it was to get the manufacturers to even talk to them about file support, nevermind DNG, thus the early limited file support. I can't understand the attitudes of Nikon and Canon, but then, I've always been of the opinion that the more accessible stuff is, the more market share you're going to get. Apparently I'm horribly mistaken, but whatever.