I'm working up modified NPC rules for my pathfinder game and agree with your wanting some way of measuring the skills of the background characters that keeps everything on the same scale. But I also like the 4e-ish idea of letting the behind the scenes rules differ from the PC ones whenever its helpful.
So, I'm putting the rules together so that run of the mill NPCs don't get more hit-points or bonuses to hit as they gain non-adventuring/not-combat levels. (Wow! did you see how the full professor, partner at the law firm, master jeweler, and excecutive chef beat up those four trained first level fighters at the bar last night!)
The biggest issue i have with the behind the scenes style of 4e is that its such an invitation to railroading. If you want the players to know something you set the DC at something easy and if you dont want them to know then you set it to something hard.
whereas with NPC levels you set them up ahead of time (usually) and have an objective skill rank or DC based on the image of that NPC's character rather then on the desired outcome.
Which is to me a preferable way of doing things.