Nice work on the photoshopping. Though it's quite thick. The current MBA is currently only 0.76 of an inch, while the MBPs are exactly 1.0 inch thick. The MB is only 1.08 inches thick, but that mockup looks like it's about 1.5 inches. With the MBPs having a bigger footprint than the MBA the proportion should be too different from the MBA in terms of perceived thickness.
Also, the curving of the case bottom is only possible because it has no optical drive. I know you put one in there but there would be some technical issues with the drive having to be placed farther in the machine or have a specialized drive made that allows the input side to be ultra slim.
The other is the lack of ports. Something that is required on a MBP. Perhaps the back of the device could be more squared off with a special hinge that allows for access when the screen is in full tilt, but I'm betting Apple will keep the ports on the side.
Since the Montevina/Penryn will reduce power usage and thus heat output considerably Apple can have smaller heatsinks, fans, and venting. This means that they can get a little slimmer if they wish. But I think the main change may be the illusion of thinness that uses a hybrid-bevelling solution similar to the MBA. In other words, instead of a 0.16 inch thickness on the edge, Apple could use smaller edge thickness than the current MBP. Say, 0.50 inch so there is enough room for all the ports. Then bevel gradually like in the MBA before hitting a the requisite thickness for the main components.
Of course, in order to get any substantial edge thickness Apple will have to forego using the bulky DVI port. They could use the the same thin, proprietary display port on the MBA, but I doubt they will. There other options are to use HDMI (which they probably won't use) or use DisplayPort which has some pros and cons of DVI/HDMI video, but has the benefit of being able to to connect to DVI and HDMI ports.