I've been wondering about NPC classes as well. Obviously WOTC hasn't released any actual answers yet, but there are some clues.
First of all, monsters have roles the same way PC classes do. Different Design and Developments & Playtest Reports have referred to monsters as Brutes, Soldiers, Ambushers, and Skirmishers, along with Leaders and Controllers like PC's. It's also supposed to be easier to make 4e monsters weaker or stronger with a simple system. From the combination of those two hints, I think it's likely that these "monster roles" will work like NPC classes do now, only even more broadly. Each specific type of monster will have a role and a level, as well as a few unique abilities that make it special. Solo monsters (strong enough to take on a whole party of their level alone) will probably be different, but standardizing the mook monsters except for a few signature powers is so DM-friendly that they must have at least thought of it.
Basically, this means I think in 4e the city guards might be level 2 soldiers, while the elite dwarven infantry are level 6 soldiers with a couple cool feats or racial abilities. This is pretty familiar stuff, but instead of only applying to humanoids, all monsters might be this way. For instance, the wolves in the forest might be level 1 brutes (with a couple special powers that make them wolves) while the trained goblin war-wolves would be level 4 brutes, but with the same wolf powers as the wild ones, because they're all wolves.
The other side of the coin is the recent revelation of rituals for non-combat magic. Personally, I think this has the potential to be great, although obviously I won't know until I see finished product. If I were designing a fantasy RPG, though, it would be an easy jump to create NPC classes for each power source that only have access to the out-of-combat ritual stuff.
So, there might be NPC priests who can't wear armor or use in-combat healing, but they can use rituals, so this means you can go to their temple and buy a ritual for restoration, or resurrection, or curing a disease, but they're still not the kind of folks that can cast a spell in three seconds to blast their enemies with fire.
By the same token, a mage's guild in a city could do business using rituals for divination, magic item creation, removing curses and things like that, but be totally useless at throwing fireballs while people are trying to stab them. For that, you'd need someone with such unusual insight into magic that they can use it instinctively and instantly, i.e. a wizard.
I have no idea if they'll actually do it that way, of course. It depends on how they create the rules for rituals and OOC magic, and how interested they are in creating a fantasy world as opposed to a fantasy combat game. ::shrug::