VideoLan says it will soon release a major 2.0 release of its open source VLC media player, which will contain many new features of interest to OS X users. The new VLC will support Blu-ray playback, if you have the proper hardware, a full screen mode in Lion, and a new interface that looks similar to iTunes.
The 2.0 version is currently available as a release candidate, but it may, of course, still have some unsquashed bugs.
Steve Jobs was famously against building Blu-ray support into the Mac, calling licensing issues a "bag of hurt." The Mac does support internal and external Blu-ray drives for data storage. With the VLC software, those drives should be able to play Blu-ray movies.
There have been some 3rd party Blu-ray software solutions, but this will be free, and we hope, reliable.
Update: There is a bit of confusion out there (both us and readers) about the playback of commercial or encrypted Blu-ray disks by VLC 2.0. I talked with Jean-Baptiste Kempf, one of the developers at VideoLan. He says both Lion and Snow Leopard can play unencrypted Blu-rays, but for legal reasons VLC can not ship the 2 libraries and the keys needed to play the encrypted discs. He says that users will have to locate them on their own, and if present, VLC will play the discs. Of course, if you have the software to rip the discs, VLC will play them directly off your Mac.