I have a macmini as an HTPC always attached to my Samsung LCD TV by using a DVI to HDMI cable. As my screen's native resolution 1360x768 is not supported natively by OSX, I use DisplayConfigX software tool to have this resolution. After upgrading to 10.5.7, 1360x768 is not supported by the system anymore. I tried both DisplayConfigX & ScreenResX but the problem remains. I found on the web this article that mentions the problem too.
http://theappleblog.com/2009/05/15/how-to-fix-os-x-10-5-7-dvi-hdmi-screen-resolution-issues/
http://theappleblog.com/2009/05/15/how-to-fix-os-x-10-5-7-dvi-hdmi-screen-resolution-issues/