Why do I say that? Well, two reasone come to mind:
1. You need a proper virtual conference space built to do this - now perhaps Stonefield or a similar co. might do this as part of a sponsorship deal and that would sort that one but the second reason
2. is that there is very low general awareness of virtual worlds, never mind the use of them as virtual conference venues.

I guess it depends on who your target audience for the conference is. If it is Joe Consumer, then it would be very difficult but if your audience is a more tightly focussed group, they could be educated on the virtual conference idea and it might work.

Could such a conference be held virtually and would the experience be good enough for attendees to benefit?

The key researchers and therefore best available speakers are located on the west coast of the USA. A virtual conference would be the easiest(and most carbon efficient) way to hold such an event. Would it work?

We’ll launch a great project about sustainable development in a near future. I’ll tell you more as soon as possible.

PS : As I guess your readers are mainly english speaking smart people, we’ve an english version of Stonfield InWorld’s blog just here :
http://virtual-worlds.stonfield-inworld.com