A US Government report that was published in May 09 is worth reading. It is a meta analysis of 51 studies on the impact of web-based learning. The conclusion is a tentative ‘it works better’ with blended working better than straight online.
In fact there were three significant variables:
Effects were larger when a blended rather than a purely online condition was compared with face-to-face instruction; when students in the online condition spent more time learning than did students in the face-to-face condition; and when the curricular materials and instruction varied between the online and face-to-face conditions.