My thoughts on that subject are a bit mixed. Before online high school I was home schooled, and I'm certain I would've learned more had I remained home schooled.
However, as an alternative to a brick and mortar high school, I think the idea has a lot potential still to be unlocked. The high school I attended (Stanford's EPGY Online High School) botched the concept, for lack of good technology. Thus far online HSes have generally hobbled along by using corporate teleconferencing software--a clearly deficient solution. But with tailored software the experience could be very good.
In fact, it's my personal view that online education + OLPC or similar initiative could do wonders for third world countries. One of the biggest obstacles to education in, say, Africa are the lack of school buildings and the lack of local teachers, according to my friends who've done peace corp. Both of these problems are mitigated by online education assuming you have internet infrastructure. Since internet infrastructure is a major component of recent investment in that part of the world, I think it would be achievable.