Looks like Apple is serious about provisioning and developer certificates for Mac apps from their latest Mac OSX version (10.8 - Mountain Lion). It has always being the case for iOS apps but not for Mac apps. Just got this error while trying to install Fink
However the good news is that there still exists a work around. Unbelievably simple one. Just right click and open instead of double clicking. Doing so Apple allows you open it if you still wish to (even after the warning about it's authenticity).