Curtains

There's nothing in front of the lens obscuring stuff, it's inside the camera. Unlike my D50, which synced with remote flashes up to 1/500th, the D300 only goes up to 1/250th. The reasons for this are technical, but basically it means that what you see here happens: even though the mirror has gone up, the curtain in front of the sensor hasn't fully opened yet. At 1/500th, it's about a quarter of the way, at 1/400th is halfway.

And as to why you'd want this? Well, if you can sync 1/500th instead of 1/250th, you can effectively eliminate half of the non-flash light. Without flashes, at 1/500th you get half the light of an exposure made at 1/250th. Now, a flash is basically a light which is on only about 1/1000th of a second. As long as that falls within the time the shutter is open, you get all that light, no matter if your shutter is open for 4 seconds or 1/500th of a second.

So the shorter you can make your exposure, the less environmental light you get, and thus, relatively, the more powerful your flash becomes. And that power you can either use to open the aperture more, drop the ISO, drop the flash power (faster recycle-time), enhance contrast between a well-lit foreground and a darker (unlit by flash) background...