Well, there's two core parts to open source and free software and neither have much to do with not having to pay for it (though that often is the case). First, there's the availability of source code and second is free as in freedom, not as in beer. But we don't yet know what license this will be released under, so we don't know what rights or restrictions may apply. Presumably, by the sounds of the text about Light Table being open source, it will be a standard Free and Open source license (GPL seems most likely to me) and he also says that the paid license will probably apply to prebuilt packaged releases, so if you don't mind building everything yourself, you may well get it for free after all.

I think the pay what you want model would be appropriate

I agree, for Free & Open Source Software, pay what you want seems like a very suitable model.