ES5 has been in the works for a long time already. Work began on the engine (it is getting a new engine, not the same Oblivion/FO3 engine) and the basic outline of the game before FO3 was even released. After FO3 was released, a significant portion of the staff switched over to work on ES5, with only a few small groups of artists, writers, and scripters left to work on FO3 DLC and patching. I haven't heard anything concrete about release date, but I would personally expect it to be released sometime from mid-2010 to mid-2011.
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