I tried searching for them here, and i cannot seem to find an official thread for them, and when i search on the MC Wiki nothing comes up D: I cannot seem to find anything about them!!!! Please tell me what they are if you know, i am very curious.
Basically, Dragon Tombs are an EMC-Exclusive "Secret Project" being worked on by the programmers that work for EMC, like Aikar.. It has been an ongoing project to remedy the in-fighting over the Ender Eggs. Hence why the later SMP servers had their Ender Dragon slayed by Shaunwhite and ICC before going live originally.
Just to add a little, the Dragon Tombs will be team based Ender Dragon fights with supplies provided inside the arena. The prize will be a dragon egg for each participant.