I haven't seen a single real info about this, only smalltalk and speculation.
But, if this (unlikely) happens to be true, only thing that I care about Obsidian is their "writing" part of the company. With id Software's technical backing and input, Zenimax financial and Bethesda Softworks distribution backing, they have a potential to... rule the world of games if they want to. I mean, I really don't know what they couldn't do (if everything working as it should) in that respect.
Of course, this is best case scenario, but I actually don't see any big inherent danger for Obsidian if they are to be bought by Zenimax... If latter continues to function as it was in the past. Thing that I don't know is what would be the reason for Zenimax to buy this frankly, tiny company. It's maybe better idea just to buy off writers and allocate them wherever they're most needed or offer them a job they prefer.