Twitter Considers Moving Its Headquarters To Brisbane
techcrunch.com
techcrunch.com
That said, I don't think they'll move. I think they're bluffing to get San Francisco to throw some of those tax breaks their way. It'd really annoy employees who just bought houses in expensive San Francisco so they wouldn't need to commute, then being told they need to commute (or sell their house in a bit of shaky real estate market and move down to Brisbane).
That being said, I'd think staff would hate this idea in general.
Several companies in the valley offer this (including mine).