That is an idea that I have bounced around a bit in my head in the past, so I am happy that you have suggested it as well. Great minds thnk alike. I think that it is one that can really drive traffic and recognition to the forums. One issue that I would like to solve before doing that is to get more people active in the forums and asking questions. Due to the nature of community credit, we have many more experts than beginners, thus many more people who can answer questions than are asking. If I were to have "Guest developers" moderating, they might get bored and the idea may lose momentum. I do have some ideas, such as the "Forum Fiesta" that aim to bring more people to the forum asking questions. A nice side effect to that is that it will also help bring the user groups together and the community together as well.
I would love to hear your thoughts on implementations. Do you think that the "Guest Developer" should be recognized for a week, for a month, for two weeks? I think that if the period of time was too long, it might discourage new "guests" for fear that they would not have time to dedicate to it. Maybe not. If we had more than one "Guest" at a time, would it take away from the prestige and make them feel less special? This is an idea that I would love to implement.