The following was posted in the Blizzard forums by Eyonix regarding travel to the Molten Core.

There will be a quest in Black Rock Mountain that will send players to discover Molten Core. Once completed, players will be able to port to Molten Core directly from from Blackrock Mountain. This will also provide players that have completed the quest and wish to join an existing raid group in Molten Core with a short-cut.

The complete forum thread can be found here.