Clark has a post on Heidegger and Plato, with a promise of more to come.
To understand Heidegger's "Platonism" one should turn first to his discussion of Plato's allegory of the cave. I'll not repeat the allegory since most of us are at least vaguely familiar with it. It is important to note, however, that for Heidegger, the four stages of the cave must be seen together. Truth, as Heidegger sees it, doesn't consist in forever dwelling in a world of light. Rather it is dwelling in darkness (the cave) while knowing of the world of light. Now where Heidegger parts company with Plato is over the ideas. For Plato this is the light and the ideas become more real than the beings in the cave (the shadows). To Heidegger one must note the function of the light which lets the beings become accessible. Yet it is the beings, not the light, which is real. Plato makes the mistake not only of reifying the light (treating it as a being) but then denying all other beings.