Currently svn log will display file changes and svn:externals property changes, but will not display changes made to the file pointed at by the external. For example, if a user commits a change to a file external, svn log will show that the properties reflects a new revision of the file is now being referenced, but will not see commit made to the file, even if it was done from the current WC.
It would be a very useful feature to have svn log include the log of the file external(s) as if it were a formal vile within the WC (even if it required a switch --include-externals) to enable.