I've searched ruby-talk for this topic, and the only messages I found show
the same symptoms I'm seeing: mod_ruby and eRuby compile fine, mod_ruby
runs (within Apache), but eRuby doesn't. eRuby runs fine on the command
line, though.

I have a few more clues that may let someone figure out what may be wrong.
When the line

    RubyRequire apache/eruby-run

is executed from httpd.conf, these error messages are written to the file
/var/log/httpd/error_log:

    dyld: /usr/sbin/httpd Undefined symbols:
    _rb_cObject
    _rb_check_type
    _rb_data_object_alloc
    _rb_define_class_under
    _rb_define_method
    _rb_define_module
    _rb_define_singleton_method
    _rb_define_virtual_variable
    _rb_eStandardError
    _rb_eval_string_wrap
    _rb_file_open
    _rb_funcall
    _rb_gc_mark
    _rb_global_variable
    _rb_intern
    _rb_io_close
    _rb_io_gets
    _rb_protect
    _rb_provide
    _rb_raise
    _rb_set_kcode
    _rb_stdin
    _rb_str2cstr
    _rb_str_cat
    _rb_str_freeze
    _rb_str_new
    _rb_str_new2
    _ruby_debug
    _ruby_show_version
    _ruby_top_self
    _ruby_verbose
    _ruby_xmalloc

I'm mystified because mod_ruby seems to be running just fine. Those look
like libruby.a symbols, and if mod_ruby can see it why can't eRuby?

Jim
-- 
Jim Menard, jimm / io.com, http://www.io.com/~jimm/
"Hey, wait a minute. You've got both eyes! You're no special agent.
You're just some jerk who hates my moustache!" - The Tick