This all kicks off tomorrow, September 15, at 15:00 Pacific time. In other areas of the world, that's:
• 18:00 Eastern
• 23:00 British
• Midnight for most of Europe
• 08:00 the next day in Sydney, Australia
BioShock HD and BioShock 2 HD will show up in your Steam library as separate entries at this time. While that means you'll need to do a significant amount of re-downloading, it does leave the originals still there to be returned to if you feel like some retro 800*600 gaming.