Apple changed the font smoothing preferences in Snow Leopard and removed the proper control for external monitors — their automatic setting doesn’t work properly. Text looked absolutely terrible on my Samsung LCD monitor after installing OS X 10.6 until I entered this into the Terminal:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
(You might also prefer -int 1 for less smoothing or -int 3 for more smoothing.)