I have been given notice to improve performance or be terminated in 30 days. This is probably just a precurser to formally terminating me, I don't think there is anything I can do at this point to improve the situation. Does this type of disciplinary writeup have to be signed by me to be valid? I have not signed it, nor was I asked to.

Secondly, the writeup was based on negative things I've said about the company and co-workers...(I stand by my words, by the way, the place is horrid), the writeup says this is deterioration in my performance. I know in California they can fire me for anything including breathing and walking, but if I go for unemployment, they will surely try to deny me on the basis of cause. If I never signed it, how could they prove I was given proper notification, if any? This is for sunny California.

I don't think you have to sign it. It just has to be in your file that you were made aware of it. If the situation at your office is dangerous or illegal you did the right thing by bringing it up with them. It is important that you do not become injured at work because then you might have to file a workers compensation claim. Your boss sounds shaddy and probably does not have the proper legal posters telling you about your rights as an employee in California. It is the law the poster describing what to do if you are injured to be displayed in a spot that can be seen.
If illegal activity is occurring please take full advantage of our whistle blower laws.
Whatever you do don't quit it is very difficult to fire someone and have them not recieve unemployment unless you abandoned your job or stole something or did something really obviously bad.
In the meantime stop saying negative things about your office and when you go on an interview don't say anything bad about your last job. Bringing up a negative about them makes you look bad too.