Because of a recent change in the file:
man page listing (an other things) are illegible in xterms with dark
background colors. The relevant lines are:
Commenting out both of these lines (as they were up through RH 6.2beta)
fixes the problem.
dark backgrounds are not our default, so you have changed the default
configuration and therefore need to adjust more than one thing to get your
desired results. This isn't necessarily a bug; many people were happy that the
new xterm app-defaults file provided enhanced coloration.