I've never noticed myself one way or the other, but I'd guess that if the animated menu preview is playing iDVD sets whatever flag it is that prevents display sleep, just like when you're watching a video in another app.
Assuming that's the case I don't know about any workarounds. If you're doing the asset encoding while saving to a disk image (so you get a progress bar, rather than background rendering) it might help--I seem to remember the screen sleeping in that situation, though I could be wrong.