Current syntax coloring options are based on what can be easily recognized by the kind of rules a lexer uses. Coloring functional identifiers requires much more, the editor would actually have to parse the statements. That's an iffy proposition while you are entering code into the editor, especially when that code contains bad syntax or typing mistakes. Some of this is actually done by the IntelliSense parser but that's a feature that is disjoint from the basic editor implementation.
Changes are coming in VS2010, its extensibility model adds support for custom syntax highlighting through MEF plug-ins. To what degree this will be used for default coloring of a specific language isn't that clear to me. I don't see any hard promises yet.