The Blu-ray Disc Association has mandated that all players released after October 31, 2007 must support a specific feature set for BD-Java—the technology responsible for giving Blu-ray discs the "extra features" like movie commentary and picture-in-picture. There are two issues with this order.
One. This new feature set for BDJ is not the one currently supported by players already on the market. That means old discs may not work in the new players, and new discs may not work in old players. That's fine for new players—there's few enough discs out that player manufacturers can make accomodations for and follow the old specs for those discs.
Old players weren't mandated to have Ethernet ports on them, like the HD DVD spec. This means that the only way to upgrade the players is through possibly discs, or sending the players back to the manufacturer. Both unwieldy. However, since the most popular Blu-ray player, the PlayStation 3, has an Ethernet port and gets regular firmware updates already, only the earliest of adopters will be screwed.
Other requirements for BD players in the fall are a minimum 256MB of persistent storage, and 1GB of storage for ones with network support.
However, it doesn't seem like network support will be mandatory, which is a shame.
Blu-ray Disc Specification Change Threatens Current Players [Daily Tech via Slashgear]