I just wanted to throw out two other options. They don't meet HowlerKarma's requirements fully, but as best I've seen, the science is sound, the information is well presented and the cost isn't so outlandish that you'll feel compelled to stick with it no matter what.
Ck12.org- Both middle and high school level science. I've spent more time looking at the middle school. No live science labs, no live teacher, but essentially a well written textbook, with a teacher guide (on request) and interactives, questions and videos imbedded.
UCScout.org- Free if you're in California and looking at self-paced, also comes with the option of hiring a local teacher to proctor or to pay for the UC Scout teacher to proctor it. UC a-g approved and fairly well designed. We are using it for math and my son really likes it. I find that it doesn't have enough practice problems, so we alternate between Khan and Scout.
