According to this ABJ story, Austin's CBD vacancy for office space is at 16.68%, and continues to drop. I would hardly call that dismal.
As a matter of fact, any less vacancy and you're looking at a market in dire need of new construction. Austin has a very healthy office market; with that said I agree we're not likely to see any office construction anywhere of 600,000+ square feet unless a major company is relocating.
http://www.bizjournals.com/austin/news/2011/04/08/austin-office-vacancy-rate-falls-to-19.html