I don't think it's what Jung had in mind
Adaptive Path's concept of a "persona" is quite similar to Joel's idea of incorporation of characters into his functional specifications.
Basically, you create stereotypical user (or users) of your product, give them names, occupations, educational histories, technical skill set and hardware, and maybe dig up a picture of them from Corbis.
The goal is to design the product for your personas. When you plan a feature, you can ask yourselves, "How would Joe want this designed?"
It's apparently pretty amazing how much consensus you can build when you are no longer designing the product for yourself, but for the same (fictional) people.