Since Skyrim is built in an entirely new engine, it would be surprising if the older editors are being used. Maybe with some hex editing, the differences in file formats can be found and accounted for but that too is surprising so soon. Are the bulk of these mods focusing on system elements that are common between games? After all, be it TES or another game, at some basic level there is always some common code.

I ask because I've got some mod ideas and if it is worth my time to sketch them out in older tools, I can get to work on them. However, I wouldn't want to dump too much work into sketches in Oblivion's engine if I'll just have to start from scratch. So, what's the word on this?

It's not an entirely new engine

Skyrim is just running on an updated version of Gamebryo, which is the engine used for all previous games by Bethesda Game Studios.

So a mod built on an old engine could have the headers in its esm or esp tinkered with to make it run in Skyrim. Kudos to the people out there doing this. But that answers my question. There should be enough similarity that I can sketch some ideas. Thanks

Yes you can do that. A few people have released mods using esm and esp files already.

*Edit. GMSTs and GLOBs haven't changed while most other record types have. GMST edits are easier to make plugins for as they can be masterless.