Could future Blu-ray media have features not supported in existing drives? While early adopters always know of the potential for obsolesce soon on as manufacturers revise their standards, normally the developers try to maintain backwards compatibility with older players. That might not be the case with Blu-ray, as a new standard is going to be implemented this year that will alter all new players:

The Blu-ray Disc Association has mandated that all players of the format released after October 31 must adhere to a specific feature set that is currently not standard for today's hardware. All Blu-ray Disc players after the fall date must support BD Java, a programming language for Blu-ray Disc media used mainly to deliver picture-in-picture for in-movie commentary and special features.
Luckily, it seems for now there are no plans to change the movie data itself on newer media, so existing players will still be able to at least play back content. The only thing they will be deprived of will be newer content and features. All hardware goes under revisions as successive generations are released, but this particular change seems to be a bit out of the ordinary. Offsetting that, however, is the ability for many players to have their firmware updated to support newer features, assuming the manufacturer will produce and release it. However, even if the value of a player originally purchased for $1000 is only a tenth of that a year from now, it'll still be a painful reminder to the consumer of the drawbacks to early adoption should the manufacturers decide to ignore them.