Beside a UDF 2.5 driver, if the content is protected (pretty much everything), then you would need a player that understands ACCS (see:
http://en.wikipedia.org/wiki/Advanced_Access_Content_System ) and for right now that means you are stuck.
It is extremely weird to this day that Sun is in the bluray consortium but doesn't provide bluray playback (even for a fee) for their own OS.
BTW, with UDF 2.5 support, HD DVD titles would also be accessible, but again with the ACCS limitation.
Once the content is on your hard drive, decrypted (assuming you can get to that point), all you need is mplayer. You'll need a good video card with a supported driver (usually that means nvidia) and a fast enough CPU (core 2 duo of sufficient clock rate), particularly on VC1 titles. On MPEG2 titles (uncommon now) you dont need as much horsepower.