Any firmware upgrades for the 500 are purely speculative. Sony said that they desire to provide updates if they are able, and would also like to support ePub on the 500. But the y also acknowledged that the reality of scarce development resources always forces them to consider the tradeoffs between providing new features for the current models versus updates for the older models. Understandably, that's a tough tradeoff. Any 500 improvements going forward probably are looked at on a case by case basis doing a cost-benefit analysis. Remember that most companies provide only maintenance and bug fixes (if even that). There are some exceptions like the recent Zune 2 software that original Zune users are able to take advantage of. But those cases are few and far between.
As far as Digital Editions support in the future, again Sony is working on it with Adobe, but even Digital Editions is just getting started, so the time frame of development may determine what model(s) get the feature. And no one can make any promises with the way technologies can shift so quickly. Probably your guess is as good as mine right now.