There is no additional copy protection between the optical drive and the player (over USB/firewire/SATA/IDE/etc) or any fancy copy protection on the disc itself. Instead, the content is heavily encrypted.
mplayer supports the containers and video codecs used by HD-DVD and Blu-ray (m2ts, evo, mpeg2, vc1, h264). I believe HD-DVD uses eac3/Dolby Digital Plus as a mandatory audio codec on all discs. Blu-ray has regular AC3/Dolby Digital as a mandatory audio codec, and this is no trouble for mplayer.
There is a patch for mplayer to decode eac3, but I am unsure if there are any OS X builds including this patch at this time.
So, if your CPU is fast enough and you can get the bitstream unencrypted, you can play HD-DVD and Blu-ray content on OS X (or Linux, or Windows using open-source software.) A 2.4GHz Core 2 Duo should definitely be fast enough to decode all HD-DVD and Blu-ray content given a fast enough codec; however, I'm not sure if the open-source codecs are good enough to decode the highest bitrate H264/VC-1 content at realtime yet.
The HD2400 and HD2600 are capable of performing all VC-1 and H264 decoding tasks in Windows using the right drivers. I don't think these capabilities are used by OS X yet.
You should be able to play most HD-DVD and Blu-ray content in Windows Vista without dealing with copy-protection shenanigans; just add recent ATI Radeon drivers and PowerDVD Ultra Deluxe. Of course, ATI Catalyst and PowerDVD are both buggy, so there are problems and they are often related to copy protection.
If you are in the market for HD-DVD and/or Blu-ray right now and do not want to wait, I would buy a budget standalone HD-DVD player and/or a PS3.