One approach that is being tried next year to get around the lack of CS instructors in high schools is the first AP Computer Science MOOC.
I have no idea how well this MOOC will work—online education for high schoolers has rather mixed results. A number of home-schooled students are relying on college-level MOOCs for their instruction, but the drop-out rate is large and the amount of feedback they get usually too little for high school students (probably too little for college students also).
At least amplify.com got an experienced high-school AP CS teacher to teach the MOOC:
Rebecca Dovi has been teaching high school computer science for over 16 years.
She currently teaches in Hanover County, Virginia where she heads the computer science curriculum committee. She is among 10 secondary school teachers nationwide selected to pilot the new CS Principles course under development by College Board.
One of the other concerns with MOOCs, the lack of verifiable measures of student achievement, is alleviated with this course, as the AP CS A exam provides a fairly well-accepted means of final assessment.
My main concern would be whether students get enough feedback on their programming assignments to learn how to structure and document programs properly—something that is labor-intensive but essential for students to really learn the material properly. Unfortunately, that is not something easily measured on a 3-hour test like the AP exam, so even decent results on the exam may not tell us whether the students are learning as much as they ought to. (Of course, the same can be said of in-person AP CS courses—we have no guarantees that the students have learned anything not tested on the exam.)
I think that AP CS does make a good test case for high-school MOOCs—there are few places currently teaching computer programming in high school, and an online course is better than no course. Aligning the MOOC to the AP test makes it more attractive to high school students and more likely to get high school credit than a random CS MOOC.
Because there are so few high schools teaching CS, the MOOC is not going to displace many teachers using better teaching techniques.