The Sloan Foundation has provided the most reliable trends about the growth on online higher education. It's most recent report suggests that more college students than ever are taking courses online. In 2007, about 20% of all college students took at least one course online.
The estimate is that now (2009), the percentage is about 25%.
Clayton Christensen in Disrupting Class has been following the trends among high school students and online classes and has predicted that around 2012, there will be accelerated growth in the percentage of students taking courses in high school. One would guess that higher education will not be far behind, but not every higher education institution will have the infrastructure to respond to providing education in this fashion.