
Some of the best mods for Morrowind, Oblivion, Fallout and New Vegas use Script Extenders. The best known example for Oblivion is Deadly Reflex. They not only add hundreds of script commands to do things that would be otherwise impossible, but also allow modders to patch the engine's executable code in memory to fix bugs and change things, without actually changing any files (ie no permanent effects.)
All recent SteamWorks releases have used a new encryption scheme called http://steampowered.com/steamworks/publishingservices.php that encrypts the executable and doesn't decrypt it all at the same time in memory. If Skyrim used this, there would have been no Script Extender. Even if it was possible, it would have required decrypting the code in memory, which could have been considered a DRM bypass, with obvious implications, so the team wouldn't have worked on it.
So I bothered Pete Hines about it, with my usual bad timing right after Rage was released and he was swamped, so I bothered him some more, and http://twitter.com/#!/DCDeacon/status/121930489266315264:
@KivanES Skyrim will not be using CEG. Modders can breathe easy.
Whatever my misgivings about the whole Steam business, Bethesda continues to do right by the modding community, which is what brought me over to the Elder Scrolls in the first place. Thank you!
I'd thank Pete myself, but I promised to leave him alone if he answered.

(Now let's hope the Script Extender team actually writes it!... pretty sure they will.)