Literally, a pro user is someone who uses their computer to earn money. If your computer is a business tool, then you're a pro user.
For software, pro ideally means that this version of the program includes features and capabilities that are necessary if your final product has to "impress" in a way that convinces someone to pay you for what you've done.
For hardware, pro means that your computing time is valuable and therefore you are willing to pay more for a set-up that completes tasks quicker, allowing you to spend more time creating and less time waiting, for example, rendering.
Note that you can easily be a "pro" in any one, two or three of these categories. This was one of the minor scandals surrounding iMovie when it first came out -- that some production houses were using it for simple things like credit crawls, and when word got out that people had paid the top-dollar rates for the company to use a free program, there was some (minor) dust kicking.
Is it possible to shoot, edit and release the next big indie using only the built-in iSight and iMovie? I suppose. Does that make the person less of a pro? I'd say more of one -- but that doesn't make iSight or iMovie a "pro" tool.
For the purposes of this forum, I'd say that the "pro" in a laptop's name means that it includes hardware that may not provide a reasonable cost/benefit profile for someone who's computer time qualifies as entertainment or a hobby.
So if Apple were to come out with a laptop that can run Adobe Premier's Mercury Playback Engine (hey, Steve -- I so know you track my posts!

), that would be an excellent example of an unquestionable pro feature.