There are rumours floating around which state the main Skyrim executable (as of version 1.1.21.0) is not tied to Steamworks – or any other form of DRM – even if you bought the game from Steam. If this rumours were true, it would be the same situation as with Fallout 3, which didn't have any DRM on the main executable even though it included SecuROM on the game launcher.
Another set of rumours is that the Russian-language version distributed by 1C in the CIS is not tied to Steam. In fact, so the rumour goes, it doesn't even have a serial number usable on Steam, so it isn't registrable there even if one wanted to do so.
Testing the first rumour would require one to save the Skyrim folder and its registry keys, uninstall Steam, then try to see if the game would still run. Testing the second rumour would need someone from the CIS to get the 1C-distributed version of the game and verify whether it is true or not. Both simple test – but I can't do either of them at the moment.
Best Answer
I can confirm that the main executable is not tied to Steam. Launching it directly start the game without going trough the "Loading game" steam window, and you can't access the steam community if you started it this way.This was surely done to enhance compatibility with outside tools like Wrye bash or OBSE for oblivion. With Oblivion on steam, you can't launch the game trough an external launcher without going trough the vanilla launcher that steam launches.Apparently, it was an oversight from either Bethesda or Valve, and was fixed in the first patch. (source)