I have the same problem. Launch GalCiv3, the cursor spins for a few seconds then nothing. Same result using the Play button inside steam. Check Task Manager and GalCiv3 doesn't show up in process list.
If in steam i right click GalCiv3, select properties, click Local Files tab, click verify integrity of game cache, it finds 6 files that it downloads. It finds them everytime.
Those 6 files are
SteamApps/common/Galactic Civilizations III/data/Core.bin
SteamApps/common/Galactic Civilizations III/data/GC3.bin
SteamApps/common/Galactic Civilizations III/data/Default.ini
SteamApps/common/Galactic Civilizations III/Campaigns/Campaign/Campaign_Arcea/Game.bin
SteamApps/common/Galactic Civilizations III/Campaigns/Campaign/Campaign_Contingency/Game.bin
SteamApps/common/Galactic Civilizations III/Campaigns/Campaign/Campaign_Return/Game.bin
Nothing gets generated in My Documents/My Games/GalCiv3/Debug.err
I tried renaming to My Documents/My Games/GalCiv3.old and a new GalCiv3 folder gets generated when I try to play.
My Documents/My Games/GalCiv3/Gfx/planets/.. all 3 folders inside are empty
The game ran fine before the patch on 7/10.
Win7 64bit, 6GB Ram, AMD Radeon 5970.
Windows and Steam are on drive C: , SteamApps/ is on drive F: , windows cache is on drive E: , All drives have > 30GB free