Long ago, before Intel had stolen Apple's heart, there was a time when a certain company's line of "Pro" notebooks consisted of three model lines. These three PowerBooks were differentiated by screen size and, at the time, they were 12-inches, 15-inches, and 17-inches.
Fast forward a few years to Monday's WWDC '09 keynote, Apple surprised us all and moved the 13" aluminum MacBooks up a few notches in the product line. As we reported Monday, the 13" aluminum MacBook now has a little "Pro" attached to the end of it. Thus, the differentiating factor between the MacBook and MacBook Pro line is (again) based on building material (plastic or aluminum). With the disappearance of the ExpressCard slot from the majority of the Pro line (except the 17"), even the expandability story becomes similar across the model lines.
Some will try to tell you that Apple has muddied the waters and tarnished the "Pro" branding by re-badging the 13" aluminum MacBook as a professional model. I, however, disagree; I believe that up until yesterday, the MacBook waters were muddy and Apple has finally cleared things up. It makes perfect sense that Apple would include the 13" aluminum MacBook in the Pro line -- it fits right in with the Apple "rule of threes" -- given that there were previously three models of professional notebooks.
It has been said that one of the differentiating factors between the professional and consumer laptops was the presence of a dedicated graphics card. If you'll recall, however, the 12" PowerBook G4 actually featured an NVIDIA graphics processor with shared virtual memory from the main system. This is extremely similar to the 13" MacBook Pro and the entry-level 15" MacBook Pro. These two models also feature NVIDIA processors with shared virtual memory, although they do lack the dual graphics chip capabilities of the higher-end models.
My belief is that Apple was wrong for ever releasing the 13" aluminum 'Book as anything other than a professional model. Ever since the Intel switch heralded the death of the 12" notebook, I have longed for Apple to provide a replacement. When the unibody line of notebooks was first released, the 13" MacBook Pro of my dreams had been born. It may have been missing a few important letters at the time but Monday's WWDC keynote address rectified that problem.
As Christina so eloquently put it, "It used to make sense to differentiate between a consumer-focused laptop and a laptop aimed at professionals... Continuing to brand nearly-identical products differently doesn't make a lot of sense." I agree with her on this, which is why I believe the re-branding of the 13" MacBook is a good idea. In my opinion, the white, plastic MacBook and the aluminum MacBook falling under the same moniker was a little non-sensical. Moving the aluminum MacBook up to the Pro line was a logical step for that piece of hardware.
Further, I think the fact that there is only a single model in the MacBook family points to the possible release of a future product slotted below the current 13" MacBook. Whether that is the fabled Apple netbook or the iTablet is yet to be seen. It is simply my opinion that the single consumer product rationale in the MacBook line is not going to last.
When you're out there mulling over the question of whether Apple's professional products have any significance in their name or not consider this: it doesn't matter. That's right, at the end of the day it all boils down to choice. Whether you think Apple made the right call or not is pretty insignificant in the face of having several different options from which to choose.
Update: As noted by commenter Tom , the PowerBook G4 never used shared system memory in any model. It had a dedicated GPU and dedicated VRAM whereas the "entry-level" MacBook Pros feature only a dedicated GPU and shared RAM. It is worth pointing out, though, that the iBook G4 also featured a dedicated GPU/VRAM combo which made the iBook G4 and entry-level PowerBook G4 very similar in terms of graphics-processing power.