I have the world's slowest blu-ray player, an original Sony. BDP-S300, I think. It lacks both ethernet and performance. Sadly, the Raspberry Pi lacks SATA, which is what the unit uses to connect to the optical drive. That gives it hack value, though not with R-Pi. I'd probably have to shoehorn something Micro-ITX in there, or use a laptop motherboard, as the optical drive is smack in the center.
I've bought just one Blu-Ray movie because the player is so godawful slow and I have to use a crappy remote with it. Ideally I'd be able to run XBMC on Windows (XP or 7) on whatever I stuffed into the case, and then I could use the android remote app. Right now my entertainment system is a mk908 running Finless 1.4 or so and it leaves a lot to be desired. Playing discs is one of those things.