All it ever displays is "The XML file does not appear to have any style information associated with it..."
I'm getting this when I run "yelp somefile.xml" from the command line, on FC4test1, on an x86_64 box. The file is valid XML DocBook. yelp 2.9.3-3.
I hear that this may get fixed by a mozilla rebuild later this week.
Seems to be fixed as of the last yelp rebuild.