Civilization IV was a highly moddable game, and many great mods/games were built off it (Fall From Heaven 2 being my personal favorite).
Does Civilization V have the same opportunities for modding that Civilization IV offered?
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As I recall, Firaxis has discussed plans to release an SDK for Civ V (as they did with Civ IV), but I do not believe they have released one yet. If and when the SDK is released, you will be able to mod Civ V like there's no tomorrow.
You can be reasonably sure this will happen--mods are a big deal in Civ V, and the game even has a built-in mod browser.