I was so glad to hear that Mike and Mark were against the "please hold your questions until the end" disclaimer at the start of a presentation.
I recently gave a series of presentation to the employees in my department and after I gave them a quick introduction I added the disclaimer that I welcomed questions. It sounded like this, "Lastly, I welcome your questions and ask that you [b]not[/b] hold them until the end. I'd be glad to answer your questions as they come up because I feel doing so will benefit the entire audience. If your question is answered by the next slide in the presentation I might say so and address it in just a moment."
I now wonder if the last part was needed.