No, we're not talking about skill subsystems. We're talking about how different character types can interact with different types of non-combat encounters. It's much broader that just having some background skills available.
Yeah, I started off to head in that direction, but then kind of lost the point as my mind started wandering about a game of PC magic item merchants.
I think you and cadfan are on the right point though, that non combat roles are tenuous to define and not really valid. The traditional roles have melded with the system -
The Face is not one character, its situational based on the skills of the PCs. Several could have nice diplomacy or bluff, a different character could be very streetwise, and the go to guy for that.
The Skill Monkey is not as needed, as skill use is spread well across the board, while still leaving the rogue with the most skill options.
The Fighter-who-can't-do-anything-but-fight is a dead concept, thankfully.
The wizard who knows things is still quite valid. Any knowledge areas he has are likely higher than anyone elses due to his Int. But other characters, including the fighter, know things as well. I make ample use of the circumstance bonus here. In my tabletop group, the cleric of Kord, the Eladrin wizard, and the dragonborn paladin all have history. I give circumstance bonuses to checks that fit into areas of their character background or class/race, with the wizard collecting these more than most, as he's well traveled, educated, and studious.
The trouble with non combat roles is actually defining them and differentiating them. D&Ds never really had this outside of the basic face/knowledge/skill monkey thing, which isn't really some rigidly define system of roles, as some have been claiming.
If you could go about it, you could do define the roles by feats. A character that wanted to be the guy in an urban environment could take a feat like this -
"The Guy" prereqs: trained in streetwise and bluff or diplomacy, cha 12.
The Guy knows his way around a town. He knows where and how to see and be seen, he's comfortable dealing with local magistrates, trademen, shady characters, all the variety the fantasy city has to offer. He understands the workings of a city, has an intuitive grasp of how cities are often laid out, how the commerce of a city functions, what wheels to grease, how to approach the locals, and speak their language. He's a social chameleon, equally at home among dockside sailors or a banquet in the Duke's manse.
The Guy gets a +2 feat bonus to streetwise and social skill use in an urban environment. Alternatively, the bonus could be 1+Cha mod. This could apply to other skill checks made in an urban environment at the DMs discretion (such as applying the bonus to a acrobatics check while dancing with a Lady and attempting to pick up the local dances on the fly).
Might be difficult to come up with a whole series of these, but it's a start.