The "new" unibody design is machined from billet aluminum, the production of it started in 2008. Seeing as how each machine used in the various steps to machine the unibody cost a few million dollars each(there are probably more than one of each, since there are probably more than one plant building them), it would be stupid(read economically unviable) for Apple to simply ditch them in favour of a case redesign that would add nothing of value to the current offerings. The previous design(non unibody) came to us from the powerbook days(back then Apple still used PowerPC processors) It lasted 7 years, from early 2001 all the way to February 26th, 2008. The unibody design came out on October 14th, 2008. The current design is thus, not yet even 3 years old. Absorbing the cost of million dollar equipment in billion dollar facilities takes some time, thus we are not likely to see any major design change to the MBP line in the near future. Take that as you will.