Bethesda has now confirmed that the main modding tool for The Elder Scrolls 5 Skyrim, the Creation Kit, will be available from January 2012. "PC players will be able to download the same development tools we used at Bethesda Game Studios to create Skyrim", Bethesda confirms. They are currently also working on tutorial videos and a new Wiki to support the modding tool and get the modders going with information.
Bethesda also speaks of an integration of the modding tool with Steam Workshop, which means modders will be able to upload their mods to the Steam service and anyone can then download and update them from there. You will still be able to upload your work to the traditional modding websites, like ModDB.