I haven't read all the posts after the first page, but from my initial impressions I agree with what everyone else has said.
First off, I am a teacher, and while we don't have an official open door policy at my school, we do try to be welcoming to our parents. After all, we have a great bunch especially for a low-income neighborhood. They try to be very involved and supportive of our school, so why wouldn't we want to welcome them?
I had a similar situation happen, and I can't say I reacted *much* better than the teacher you are discussing. Granted, I didn't give a look of pure hatred, or discuss it with the parents afterward (there wasn't really a change), but I am fairly certain I didn't handle it as professionally as I should have.
My students were working, and my door was open (we keep our doors open unless there is a lockdown), and a parent just walked in and started talking to her son. A) I had her son working on something independently, B) It was one of the few times I was demanding silence from all of my students (I like a loud classroom in general, so this is rare), and here is this woman, coming in, disobeying the rules I had set-up for the situation, without so much as a hello/knock/"May I come in and chat with my son for a couple of minutes?" type of introduction.
My reaction was to pretty much ignore her after I gave what I am pretty sure was a look of, "What the heck are you doing woman????" I have to admit, I still have trouble dealing with this parent after that first impression.
On the other hand, I do believe I should have handled my situation more professionally, (Perhaps a, "Can I help you?" and then a short but friendly conversation afterwards saying that if she could just check with my before entering the classroom to ensure she isn't interrupting something important, as I have a couple of students who have difficulty focusing) and if she had a problem with that, I can refer her to my principal. And basically, if I feel I should have been much more professional, you can just imagine how poorly I think your child's teacher reacted.