Many are having issues with font smoothing not working correctly on external displays with SL 10.6. I researched this issue and found this blog posting: http://blog.jjgod.org/2009/08/18/snow-leopard-vs-dell-lcd-displays/ the blog itself discusses the issue, and the fix is found in the comments. Basically, for now the fix is to open Terminal App and enter: “defaults -currentHost write -globalDomain AppleFontSmoothing -int 2″ without the quotes, and hit return. a log out or restart might be required. this fix seems to stick even after re-boots and shut-downs.