I too agree with the ClearType sentiment. I know font appearance is highly subjective, but to me the Silverlight text looks awful on all of the monitors I've seen it on. (It reminds me of Safari text which looks just as bad.) I've tried all kinds of font/size combinations in Silverlight and they all hurt my eyes. Light text on dark background may be somewhat better.
I can understand not using ClearType on Silverlight for cross-platform purposes, but if that's the case, please consider disabling your smoothing algorithm altogether.
Also, does the existing algorithm make any "subpixel" assumptions? If so, it's going to vary (sometimes quite significantly) from monitor to monitor and won't make much sense without being user-configurable (like ClearType's tuning wizard).