The Creation Kit, as it's called, was mentioned in the flood of details recently revealed but it wasn't clear what exactly it would be.
A modding kit seemed inevitable, really, as support for player-created content and mods has been a hallmark of Bethesda RPGs. Everything from UI tweaks and new quests to massive graphics overhauls and brand new, full-length campaigns have been created for its games by fans. It'd be mighty difficult to turn away from that sort of support.
Skyrim is slated to launch for PC, Xbox 360 and PlayStation 3 on November 11, 2011--11/11/11--though you shouldn't expect mod support on consoles.