Well Valve is my favorite developer partly because they have the leeway to do whatever they want and that's part of what makes their games so damn good. Of course, thinking this, my knee jerk reaction is to say that any buyout will immediately lead to a degeneration of the quality of their games.
On the other hand, I've only played Fallout 3 from Bethesda, which I freaking love. (I know I know, Oblivion and all that but FO3 just piqued my interest immensely so I bought it) Going from that game and what I've heard as a whole from people about that company, I'd say I'm okay with this. Assuming that ZeniMax doesn't mess anything up.
I'd still like Valve to be independent but if they had to be bought out, I'm okay with this.