Journalists owe loyalty to the public, or, I guess more specifically, to their readers. Loyalty to the public is an integral part of being a journalist, maybe even part of the definition of the profession, since first and foremost a journalist's job is to correctly inform the general public. After the public, I think journalists owe loyalty to their paper; they should work hard and honestly for their employer.
I think it's a lot different for PR practitioners. PR practitioners owe only the most basic, undeniable loyalty to the general public. Once they are employed, they owe their loyalty to the company. If being loyal to that company goes against their other values, they should quit, but while PR prac.'s are employed, they owe it to their employer not to hurt or be disloyal to the company.
Loyalty ranks really high on my list of personal values, but only in a very specific way. I believe in being loyal to myself, my family, and anyone I have made a commitment to or with. I don't believe in loyalty to causes, and I feel only the barest loyalty to the general public. I don't think I owe the general public anything other than not infringing upon their rights and liberties. This is a reason I would make a terrible journalist. I probably wouldn't want to tell the public a lot of what I was hired to write (I am thinking of an essay by Emerson, I believe, in which he declared the news gossip and useless noise); however, I do believe very strongly in the free market of ideas.