Bethesda owns the rights to your Skyrim Creation Kit mods
Some digging into the newly released Skyrim Creation Kit's End User License Agreement has revealed that all Skyrim mods made by using it effectively become the property of Bethesda as soon as they're distributed.
The Skyrim Creation Kit EULA reads:
If You distribute or otherwise make available New Materials [Mods], You automatically grant to Bethesda Softworks the irrevocable, perpetual, royalty free, sublicensable right and license under all applicable copyrights and intellectual property rights laws to use, reproduce, modify, adapt, perform, display, distribute and otherwise exploit and/or dispose of the New Materials (or any part of the New Materials) in any way Bethesda Softworks, or its respective designee(s), sees fit.
In short, Bethesda reserves the right to use any user-created content or mods in any way it sees fit - including modifying, adapating, and even selling it. By using the Creation Kit you are basically giving away all your rights to the mod and would have no way to claim royalties.
NowGamer, who originally reported the EULA, heard from Bethesda who said this:
These are standard T&Cs. We have the exact same language in our previous kits. If you install the Oblivion Construction set or the Fallout 3 GECK, you’ll see the same language in the EULA. We release modding tools because we want the community to be able to create new content within in our games.
While it may seem wrong, this type of agreement is hardly unheard of. Modding tools for Unreal Tournament and Crysis 2 contain similar terms with their EULAs which put strict rules on non-commercial distribution, retaining copyright of existing assets, and banning offensive content. But both stop short of waiving modders' rights.
There you have it. If you want to use the tools, you've got to give up your rights to claim royalties. The good thing is, the majority of modders do it for the love of the game, not for the money.