It's most definitely a learned skill. I work as a trainer and people new to the job go on a week long Presentation Skills Course which basically teaches them how to stand up in front of an audience and communicate with them. It is amazing to see people change during that week from stammering wrecks into people enjoying themselves in front of - and with - an audience. Of course like anything else, some people are better than others and some people hardly need teaching, but everyone gets something out of it.