Half-Life 3 has popped up in an PDF list of exhibitors and games published by the organisers, along with Dragon Age III from BioWare.
I don't want to get your hopes up. I really don't. More than likely this is either a mistake or a placeholder, so don't get too excited just yet.
But... damn. Could Gamescom 2012 herald the reveal we've all been waiting for?
Update: Valve has spoken out to squash this rumour, telling Lambda Generation that "we will have the Steam team there to meet with developers and publishers. No game showing planned." False alarm.