I took the plunge and bought one of the new light cases at Sony Style yesterday afternoon and I'm very pleased (though I'll admit that it is overpriced...). First, the material of the case itself appears to be the same as the standard 505 case. And, as previously mentioned, it does not have the pockets that the deluxe cases have. What it does have is light!
The light panel (very similar to a lightwedge, but plastic not glass) fits the screen very neatly. In fact, it has a bevelled edge to fit over the screen. There are also two rubber feet that rest against the left side of th Reader so that it isn't smashing down and there seems to be some kind of resistance that keeps it from slamming down (not quite a spring, but something to soften the landing). It does make the case thicker, but it is still slimmer than a Kindle. I didn't notice any extra weight after a few minutes of reading.
The light was pretty well distributed across the page but it is not a completely uniform light. There are two levels of brightness available (I used the full bright in a dark room and it didn't disturb my daughter while she fall asleep) and the lower brightness worked nicely in a dim (but not dark) room. Because the light is diffused across the screen, I don't think that it will cause much problems for sleeping spouses (and it does not appear to give off anywhere near as much light as my Mighty Bright).
I did notice that the light wedge had a tendency to "fall" away from the screen when I was reading lying down with the book help up away from me, but I quickly learned to just adjust where I kept my left thumb and that problem was solved (at least until I fell asleep...).
All in all, I wish the price were about $20 cheaper (and I think that Sony would sell a lot more of them), but I think that I'll be very happy with the purcahse (and it sure keeps from worrying about losing my Mighty Bright [or having my wife ... er ... "borrower" it and misplace it]). I think that the addition of the lightwedge gives the Reader one of the key missing components and makes up for a backlight quite well.