Beyond Bullet Points has a great post today on whether it matters what you put in the heading of a powerpoint slide. It turns out that ion research performed by Michael Alley slides that include a headline type heading resulted in significantly more student learning than when slides included only a general heading.
As Cliff Atkins points out in his BBP post, that means that teachers should be writing entire sentences on their slides.
BTW: Check out the difference between the two slides in the picture to the left (Wow!).