From MacOSXHints.com…
“You can force OS X to use LCD font smoothing on all displays with this Terminal command:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
The number 2 here corresponds to Medium – Best for Flat Panel. You may also use 1 for light smoothing, and 3 for strong smoothing, as per the original OS X font smoothing options.”