According to MacRumors, reliable sources are saying the next generation MacBook Pro coming either later this year or next will be sporting an all new case design. Ilounge also reported a new case design would be saved for the following generation of MacBook Pros and not the recent Spring refresh:
Next year is the year when Apple will introduce an all new design for the MacBook Pro product family, which is already under development at Quanta in Taiwan. It’s being described as a big, “milestone” release for the Pro family, as compared with the speed bump features that will be introduced in [February's] models.
What would a new MacBook Pro look like? Every 3 years Apple tends to overhaul the case or design of a product. Future MacBook Pros would likely lose their optical drive and gain spacious flash storage which would lead to a much thinner case. Apple may opt to keep the unibody design but could turn to a different material to build the case. While Aluminum is sturdy, it often dents when dropped or hit. Lighter materials such as carbon fiber exist but don’t expect Apple to use plastic in the case design.