The Shivering Isles expansion pack has been officially announced by Bethesda and will be available on PC through retail this spring. At that same time, the new content will be distributed through Xbox Live Marketplace to Xbox 360 Oblivion owners. The possibility of Shivering Isles releasing on PlayStation 3 has not been addressed, but presumably Bethesda could include the expansion with the forthcoming PS3 release of Oblivion (if there's any space left on the Blu-ray disc) or opt for digital distribution.
Today's announcement confirms many of the details that were revealed in PCZone, which published news of the Oblivion expansion several weeks ago. Bethesda's decision to cease its silence is likely tied to an OXM feature article that hits newsstands this week. Bethesda claims Shivering Isles with feature 30+ hours of gameplay, suggesting that the expansion will set a new bar, both in scope and price, for premium add-on content distributed via Xbox Live. Horse armor this is not.