This probably gets asked of every Drupalcon organising team, so here goes...

Will it be possible to provide live video & audio from the conference, for those unable to travel?

Virtual attendance could be an option with the following components:
Live Video and Audio
Presentation available for download in advance.
IRC/Twitter for backchannel & audience questions [something similar was used at "Do it with Drupal", I believe].

While I've never actually used DimDim, the Drupal Dojo has a licence - Maybe that could be used.
http://groups.drupal.org/node/16320

As the conference will be on the US East coast, the times of the presentations means European users unable to attend, could certainly attend 'virtually',

It's probably too late at this stage, but maybe even an option to purchase a 'virtual attendance' ticket, to cover the cost of the infrastructure / manpower
could be considered.

Keep up the good work!
Alan