My advisor had some comments on this question and I am posting a paraphrased version of them, with his permission. He says (roughly):
Before talking about work in progress, one should carefully consider what the message of the talk is going to be. If the message is to communicate an idea rather than a complete proof, then this may be one situation where it is appropriate to talk about work in progress.
However, once you speak publicly about your work, other people are free to use your ideas. If you indicate what you specifically plan to do with your ideas, then it is not considered appropriate for others to work in that specific direction. But if they see ways to adapt your ideas or your results in order to apply them to something you did not say you intend to work on, then that is alright.
