I just found out that in Silverlight 4, the fonts are no longer whitelisted to the ten fonts out of the box.  Silverlight will now use any font that it can find on a machine without embedding. For example, in my RIA Services sample, I changed the font to Elysian (a font I have installed) and it picked it up:

I am told that soon you’ll be able to iterate through the fonts to allow users to pick fonts, but it doesn’t look like its included yet.

Wow…