How do you open your presentation? What a great question, and a frequent one.
And the answer no one wants to hear - it depends. Your audience, experience, timing, topic, and a host of other factors will influence what is appropriate and what can actually work. The same speech given at different venues, or to different audiences, needs to open differently.
One thing, however, always remains the same... continuity.
Your introduction, whether humorous, personal, shocking, whatever, should always relate to your topic. The example of what not to do is the speaker who takes the microphone and shouts "SEX!" and follows with "Now that I have your attention, I want to address the economic ramifications of off-shore drilling in developing countries." Sure, you got our attention. You also told us you have no respect for us as consumers of your message, and you spent no effort to develop your introduction. So we will reciprocate by giving you no respect as a presenter and putting no effort into listening or comprehending.
There are times when you probably don't need to work too hard on your introduction. If you are an M.D. and you start with "I have discovered a cure for cancer," then you already have our attention. Other examples might include the Preseident starting off with "We have made contact with life on another planet, and they request all seasons of Gillian's Island," and the Mayor of a city announcing "We have a budget surplus so great that we are refunding all property taxes for the last 10 years."
Short of those, please create an introduction that gets our attention, gives us some idea of what to expect, and actually relates to your topic.