The most information at the moment is in the Stockades, which has a post on MMO-Champion.
The Deadmines will have the bosses changed to reflect who Vanessa VanCleef has recruited.
Posted from todays The Queue: Dapper dog
"So what exactly is happening in the new Deadmines? Who exactly are these new residents? Any word on bosses?"
Helix Gearbreaker (a goblin rogue), Glubtok (an ogre), and Admiral Ripsnarl (a worgen) are the figures that Vanessa Van Cleef recruits for the new Defias Brotherhood. It's likely that they'll be the new bosses in the instance, with Vanessa as the final boss.
Obviously Helix Gearbreaker will be the replacement for either Sneed or Gilnid, Glubtok will replace Rhak'zor, and Admiral Ripsnarl will replace Mr. Smite (May he improvise in peace) and Vanessa being the replacement to her father, Edwin VanCleef.
Scholomance was reportedly the same as previously but reduced level ranged (I suspect this will change over the beta)
Stratholme will have a few changes which can be seen on its Cataclysm section, but the dungeon has been split between Festival Lane and Elder's Square, the Main Gates will no longer close upon entrance, level retuned, Balnazzar has been defeated and returned (it was merely a setback) and he killed and raised all of the Scarlet Crusade, as well as Baron Rivendare being replaced by Lord Aurius Rivendare. Also NPCs will help assault Stratholme and setup Argent Crusade bases throughout the dungeon.
Dire Maul will be retuned as a lower level instance.