There is a bug in Snow Leopard that disables LCD (sub-pixel) font smoothing on many third party LCD displays, including models from Dell, Samsung, LG, HP, EIZO, and Lenovo. Here are the details:
Snow Leopard introduces simplified Font Smoothing options under the Appearance pane in System Preferences. Prior to Snow Leopard, the available options were:
Automatic - Best for Main Display
Standard - Best for CRT
Light
Medium - Best for Flat Panel
Strong
In Snow Leopard, you can only choose between Automatic and Standard CRT. Since few people use CRTs these days, most users now have only one option: Automatic. The problem with the Automatic option is that OS X incorrectly detects many third party LCD monitors as CRTs, and consequently, disables LCD font smoothing.
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.
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Should anyone need to remove the preference, restoring the default, use:
defaults -currentHost delete -globalDomain AppleFontSmoothing
And to read the current value (a “…does not exist” error indicates the setting has been properly removed from the global domain):
defaults -currentHost read -globalDomain AppleFontSmoothing
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I just used this command to successfully restore proper font smoothing on OS X Lion 10.7.2.
(defaults -currentHost write -globalDomain AppleFontSmoothing -int 2)
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I also experimented with various settings: system default settings, completely disabled, "1", "2" . I chose a value "2", after a long hesitation:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
On Lion it's more application-basis. For one application is more suitable value "1", for other value "2". Finally, I stick with a value "2" as most suitable and universal for my NEC Display MultiSync PA271W on OS X Lion 10.7.2.