Obsidian will be doing the lion's share of the development work on the sequel, but BioWare will have a hand in the development of Neverwinter Nights 2. They have already provided Obsidian with the Aurora engine to build the game on. In addition, since they are most familiar with the Aurora engine, they will be providing technical assistance in using the engine to its fullest, and will also be looking over Obsidian's milestones and giving input to keep the project in line with the vision of the original. As Greg Zeschuk put it, "We've got a good relationship with the folks at Obsidian and we definitely want to see them succeed in making Neverwinter Nights 2 a great game; we see it -- in part at least -- as still being a BioWare project."
While the sequel will be built on the same engine as the original, it will be a highly enhanced version of the Aurora engine, and you can expect some graphical updates to take advantage of current hardware.