Just came across "Big Blue Button" which claims to be open source web conferencing for distance education. This might be a useful alternative to Wimba Classroom and DimDim. It seems to have much the same features. I joined the demo conference and it worked okay on my low power netbook. The source code is available.
However, BigBlueButton, Wimba Classroom and DimDim all use Flash and all assume that the presenter is showing Powerpoint, PDF or similar documents. DimDim does have a limited function for sharing web pages, but does not exploit this fully. There is scope for a product which genuinely works with web pages. Perhaps this could be done with HTML5 and could be tightly integrated with Moodle. This would simplify the software needed and reduce the resources needed to use the system. Essentially the system would be reduced to a small amount of code to knit together the web, audio and text chat functions.