that thing on there looks like the videocamera and laser ranger on the land worrior Future Combat system.... but the staypuft marshmellow man dont have the eye reticle to view the image in attached to his helmet... so..
I believe that he has a MILES transmitter mounted... it's the Army's version of laser tag. It fires a laser beam each time a blank is fired (or each time you pop your finger on the audio pickup -- stealth sniper-mode). It also requires a blank adapter mounted on the rifle, and sensors on the load-bearing vest and helmet.
It is an AN/PEQ-2A Target Pointer/Illuminator/Aiming Light.
Commonly refered to (phonetically) as the "Peck-two"
With the targeting laser on high power you can do some real eye damage!
Although the one in the original post looks a little funny. It appears to be a little more square than ones I have used. The switch on the top rear looks fake to me. If I was to put money on it, I would say it was an airsoft version of the AN/PEQ-2A.
You are correct about the MILES transmitter. Not only that, but the MILES transmitter mounts forward of the front sight assembly and the PEQ-2 mounts to the rear of it on the 12 o'clock M-1913 rail. You can mount the PEQ-2 on the 3 or 9 o'clock rails (or even the 6 o'clock I suppose), but that really screws with the practical application of the elevation and windage adjustments.
On a side note, the AN/PEQ-2A is made in Londonderry, NH!
It's a PEQ-2, probably a fake as those are most definitely serialized and you can't exactly just buy them, AFAIK. All the MILES emitters I've ever used mount forward of the front sight assembly onto the barrel. There could be newer ones that mount on the rails, though, and the Corps is just too poor to buy them until they can get them as hand-me-downs from the Army.