MaDogNik96 of GameFAQs reports that Drake will be looking for the "lost" ships of Marco Polo and will joined by one Chloe Frazer, who is voiced by Claudia Black of Stargate SG-1 fame. Speaking of voice actors, Nolan North will reprise his role as the main protagonist (editor's note: yay!). New improvements and mechanics, some of which were already reported, include free climbing, stealth gameplay, "more dynamic" melee combat, elimination of SIXAXIS controls, access to your weaponry during any stance, improved graphics, retooled QTE sequences that give the player more control and more.
The game will also reportedly use "nearly 100%" of the PS3's power (Drake's Fortune has been quoted as using 30%), but that sounds a stat that will remain true only until Uncharted 3 is unveiled. A scan of the article is available here. A summary of all the information can be accessed here.
Source -- Uncharted 2: Among Thieves [GameFAQs]
Source -- OMG first Uncharted 2 Scan [GameTrailers.com]
Source -- New Uncharted 2 INFO -- GI [NeoGAF]