Yep, this is by design unfortunately.
Here's how to revert it
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.