Why would they? That would be like some random developer under EA's arm making patches for BioWare games just because EA owns BioWare. And even then, Zenimax doesn't own Obsidian.
Well, Bethesda had some developers on the NV project from what I've heard...they weren't there to create content so much as to help with tech/engine stuff. I know that Obsidian was in charge of content and in-game scripting with some oversight from Bethesda.
Then again, BGS knows the engine better than anyone, so they could send some people to help them. But no way that BGS is mainly in charge of developing patches.
Not saying they're mainly in charge, but I'd be surprised if they weren't involved since the person credited for heading up QA for the game is a Bethesda employee. :shrug: