Just a quick issue I ran into today whilst trying to make use of runtime font loading inside Flash Builder. Where as before in Flex 3 it was a simple case of creating a class to construct a swf that would then be loaded in the main application. Today I ran into an Error #1014 - Class mx.core::FontAsset could not be found, when running the main application.
The fix as it seems is to add this to the compiler arguments of your font library swf.
Now if like me you use Ant Builder to compile your projects - simply add to your mxmlc tag within your ant script as shown here
Update: Adobe mention a fix to the noticeable file size increase. It appears that the increase is due to it including all classes that our "classes inherit from plus other framework dependencies" so we need to amend this script a little to something more like
mxmlc -runtime-shared-library-path=c:/p4/flex/flex/sdk/frameworks/libs/framework.swc, framework_3.0.183453.swz,,framework_3.0.183453.swf rsls/SimpleApp.mxml
For a more detail explanation check out Adobe's comments