I've been pretty lucky in my cake career, in that I rarely encounter another wedding professional who's just outright nasty. Most people want to cooperate, and generally try to work together to make the job easier for everyone. (See a post that I wrote on the topic of cooperation for Weddlady's blog here.)
I did say, though that I've rarely encountered nasty people, not that I've never encountered them. I remember one florist who liked herself a lot, and refused to let me put the flowers on the cake, since she had them all strewn around her on the floor and said that she'd just put "whatever was left" on the cake when she was done. Uh, no.
Then there's the caterer who had a reputation for screaming obscenities at other pros in front of clients. Very nice.
And the other pastry chefs who email clients and insult them after they decide to go with a different baker. Here's a hint: Telling a client that they're idiots for going with someone else, and that their cake will be a disaster if you don't make it generally doesn't make people want to hire you.
A good professional won't tell you nasty things about other wedding pros, but we know them. Let me tell you, there have been times that someone's name has come up in conversation with clients, and it was all I could do to bite my tongue and not tell them to run as fast as they could. But I don't do that, since it isn't professional. I also realize that I do tend to hear a lot of horror stories because I'm in the business, but unless I witness the event firsthand, I don't really know how much of it is exaggerated and how much is true. So I don't say anything.
I will also say that there are people who I genuinely don't know, so just because I don't endorse everyone doesn't mean that they do bad work. It could just be that I'm not familiar with them. However, when you're asking your wedding professionals who to hire, take note of whether they clam up and get a desperate look in their eyes when you mention a name, or whether they enthusiastically endorse the person. If you get either reaction from a lot of people you might want to take note of it. If someone has a reputation for making other pros jobs difficult, they're not going to make your life any easier either.