I don't think it has much to do with money. These companies either have or are already burning trough a lot of it. It is about careerists avoiding accountability. That is why you hire contractors, have open offices and host in the cloud. As long as no one can blame you for anything, which they will to further their own career, you will make it to the next level.
Plus, personnel costs (especially in Silicon Valley) are a multiple of floor space, so I can't understand that argument either.
But then, so far, I've never had someone who calls the shots on the open office model go public on why they went for it. I mean the cynic in me thinks it's about cutting costs (but then why do they have developers in SV instead of outsourcing it?) or showing off (open offices look good on pictures / when the manager walks around in it).