Yesterday I was offered a graduate teaching position for a course that I really wanted to teach. I e-mailed today to accept it and they said they had given it to someone else. I know the course director and he suggested me. I thought I had actually been offered the position so was very annoyed when it was taken away from me. Is this unprofessional. Can I do anything about it. Should I get in touch with the course director to find out what happened? Or should I just accept that GTAs are treated with no respect?

If this was done formally, there should be a paper trail in your favour. If not, then its difficult.

I was offered the position by e-mail and asked to send in CV etc if I wanted to accept the assignment, so I saw it as a firm offer if I was interested rather than having to apply with others.

Get in touch with the course director.

If they've already given it to someone else there's not much you can gain other than an apology. However, I would email and express your disappointment, if only in the hope that in future it may encourage them not do the same thing to someone else.

Very unfortunate for you. I'd be annoyed too.

I have had an e-mail from the head of department with a sort of apology but she was positive and stressed there would be other opportunities around in other departments and suggested I get in touch with other people. There is another new course which I really want to teach but I can't find out who is course director yet. Anyway, I am a great believer in fate so maybe I am destined to teach the new course which is more in my area of research.

Sounds like the best attitude to adopt to me. I'm sure something better will be just around the corner! Good luck.

and yes, many places, GTAs are simply treated with no respect whatsoever.
maybe you should feel lucky not to have the job