Are you playing on OSX or Windows, out of curiosity?
Also, it sounds like you are definitely able to bring up the Shift+TAB overlay, so clearly you did launch this through Steam and all that, right? And Steam isn't in offline mode.
You might want to try deleting all your Tidalis game files except your runtimedata/save folder and your runtimedata/settings.dat file, then let Steam reinstall the game. Maybe Steam somehow thinks it is still in demo mode or something since the game was not re-downloaded. I don't really have any insight into that sort of thing, as it's all internal to Valve, but it's curious that the few rare reports of this are all people who played the Steam demo and then bought the game.
Hope that helps!