The presentation canvas is no longer limited to the overhead projector. Laptops, TV screens are often used to display PowerPoint presentations with a wide screen or 16:9 aspect ratio.
For movies, 16:9 is great. The wider screen is more natural for our eyes. For slide design though, I find it less useful. We can read best when titles are short, or text is set in a narrow column. As a result, with 16:9 my slide design almost always changes to a horizontal story: element or title line on the left, and 1 or 2 other elements to the right. It always takes me a bit longer to comprehend the slide.
So, when I am designing a presentation specifically for a plasma screen (a trade show booth for example), I will stick with 16:9, but for other output devices I revert back to the good old 4:3.
It is always good to contradict yourself, see my earlier post from August 2008.