That having been said LE still needs PC to search your car.
Or he can claim he believes he is in some sort of danger so he can perform a Terry search. The SC has nearly given carte blanche for LEs to do this as long as they claim they feel endangered (whether it is true or not). Best bet is to lock your car door behind you when exiting the vehicle, thus requiring the LE to get a warrant.
To address the immediate topic, I understand it is good to have protection in writing from being canned for having a gun, but realistically, here are my two gripes:
1.) UPS had a gun ban on company property which I frequently violated. I never told my managers I had a gun in my car and that seemed to be the end of it. If they asked to search my car I would have told them to get a warrant. If they then responded they can't because they aren't Police, I would have asked them to get a cop, then told him to get a warrant. If they fired me for that I would have just got another job. Really, don't run your mouth about the nice piece you have in your car and the odds of this being an issue are very nearly zero.
2.) Texas is an "At-will" state, and suing your employer for wrongfully firing you is probably going to be more trouble than it's worth. I doubt this law will change much of anything, though I do appreciate its "Spirit."
With the recent shooting in AZ I don't hold out much hope for any pro-gun legislation this session, unfortunately.