I don't think in the general sense the idea will ever have merit. Consider:
- The discipline is so varied. What makes a good embedded developer has little to do with what makes a good web developer, or shrink wrap developer, or line of business developer, or data base developer, etc.
- what makes a good web developer, for example, is very different what made a good web developer even a few years ago, and accepted practices vary widely from place to place or platform to platform.
However, I feel there is a need/market/desire for a credentialing process for programmers that build systems for use in medical equipment or the space program, to name a couple examples. In short: systems where the software could endanger human life.
The problem here is that as soon as such a process is created, about 50 other places that don't really need this process will start enacting policies that require these programmers: military and other government agencies come to mind, as do things like ISO9001 certification.
Thus the market would drive up prices for these programmers. You'll start seeing a lot of people gaming the system, and if there's one thing programmers are good at, it's gaming these systems.
In the end, the credential wouldn't really matter. You'd have companies hire uncredentialled programmers for positions that require the credential, only to send them to a two week training course teaching them how to pass the credential requirements. Every CS university would have a course in how to do the credentialing. It would be meaningless, and serve only to limit access to the profession.