No one knows but only the iPhone Dev-Team and they're not big on giving ETA's out.
If you really need an unlock for the 3GS, the only option is to upgrade to the iPad baseband (6.15) and use ultrasn0w to unlock. But there is some drawbacks to doing that.
You'll most likely lose GPS functions, the baseband can't be downgraded, and you can only upgrade to a different iOS with a custom firmware.