I imagine this may work similar to how the modding worked for Unreal Tournament 3.
Modder makes the mod on PC, then ports it for the console (For UT3, a tool was used to 'cook it' for PS3) and then uploads it for users to download.
Like UT3, I imagine it'll be only simple mods that can be ported. ie: Someone adds a new town, puts a locker full of weapons somewhere or tweaks a few stats.
User made content (meshes, textures, etc) might not be allowed, may depend on what controls there are on content (ie:third party assets) and how to restrict someone from downloading a load of high res/high detail stuff onto their console that might fry it.
Anything requiring a third party tool (Script extenders, Wrye, animation updaters, etc) wouldn't be allowed sine the closed nature of the systems wouldn't allow their use.
It'll be interesting to know what sort of trouble shooting methods are going to be available for console users. Sorting load orders (It'd have to be done manually, since LOOT probably wouldn't be allowed) along with methods of removing the mods cleanly without impacting saves. Skyrim baked scripts into the save that persisted even after the mod was removed and could screw things up big time.