After some rumors and reports from various places, today bethesda officially announced the expansion to The Elderscrolls IV Oblivion.
The name is the shivering isles, and it's also the name of the daedric plane of Sheogorath. We can expect to fully explore it, become Sheogorath's champion and battle new (and old) creatures.
You can read the full press release here;Press release
The release date is set for the coming spring, and I'm sure we can expect more info in the coming months.