What topics should one cover?
What do you want the audience to get out of it? Where is the audience now?
I spend quite a bit of my time getting dev and ux folk to play nice together. I generally have three approaches depending on the context:
1) Find the biggest pain point in the way they're working together now and try and fix that. For example if folk are misunderstanding wireframes and comps, take some time to talk about some of the principles of graphic design like repetition, hierarchy, balance, etc.
2) If they don't really have any idea of what's involved in UX work give a general overview of the various sub-disciplines to break the "making things pretty" myth (IA, Graphic Design, Interaction Design, User Research, User Testing, etc.)
3) If they're building the wrong thing, rather than building the right thing wrong, spend most of your time on the more generative end of user research + user testing. Talking to users and closing the feedback loop with users are what needs fixing.
What are some common
questions/discussions that usually come up during these presentations?
It depends ;-) I've had everything from very naive questions about why you need to worry about fonts, to sophisticated questions on how user research fits into product development. I don't think I can pick a common theme.
What would be your "Most Important Message" for the audience?
You Are Not The User.
Getting past this is often the biggest step for people who aren't involved with UX work.
If they take away nothing else, understanding why user testing is necessary will get almost everybody in a position where they can start making progress. It closes the feedback loop so people can spot that there are problems, which necessitates figuring out how to fix 'em.
Finally - two tips for getting UX across to developers:
Be careful with the word "design" - since if you use it in an unqualified way developers often hear "software design/architecture", which can lead to people talking past each other. Say "interaction design" or "graphic design" or whatever instead.
You'll find it much easier to communicate if you pick up some development knowledge at the same level as the UX knowledge you're trying to put across. This means more than just a bit of HTML/CSS/jQuery :-) There are development metaphors that are useful analogies to design. It also instantly gets you more mutual respect if you are obviously as interested in understanding the developers constraints as you are in communicating yours.