Pelical mirror is a interesting idea. However, I think canon must have found out something that they do not like about Pelical. canon made the Pelical SLR in the 60's then give it up. As far as I know Canon held the pattern right on Pelical mirror in the SLR, and it has expired (just like the Leica M mount pattern). Canon tried to bring it back again in the 90's and give it up again. Does anyone know the reason behind it, besides the dimmer view finder. I am very curious to know. Now Sony is trying to use it now .
Pelical mirrors allow you to run the AF system during exposure (good for video). But it eats up about 1/3 stop of light, negligible to most users, but if you do a lot of low light stuff you may want every bit you can get.
Here is a interesting article about it: http://asia.cnet.com/crave/2010/11/04/a-dark-side-to-sony-pellicle-mirror-camera-design/