I was playing around in Photoshop with the opacity controls and discovered a weird optical illusion which I'd never seen before. So I made it in Flash. Seems to work pretty well if you're about 10-20cm from the monitor as you watch it. It's important that you don't move your eyes. Also don't try to look at the whole thing. Just focus on a point.
It usually works for me, though sometimes I don't see it. If it works, it looks like the oval fades away entirely. It's really convincing. Have a go.