While they haven't set a specific date for the update going live, Valve has been talking what gamers can expect. Most notably are unlockable weapons for the different classes and a new mode where players have to escort a mining cart through the new Goldrush map.
The planned 'Overhealer' medic unlockable has been dropped according to an email; two other mediguns will still be included with the update though.
"Goldrush grew out of thinking about Hunted... and the main difference is that we've replaced the VIP character with, er, a cart."
In Goldrush the blues must stay with the cart while it travels along to the red's base, red players can stop the cart if they get near it - all this mayhem happens within a time limit.
Clickto read the full interview between the guys at Valve and CVG.