Just in case anyone else is still having trouble upgrading to v1.8.5  
(in my case, from 1.8.4) on Mac 10.4, here is how I resolved the  
following issue.

### Issue ####
~$ irb
dyld: NSLinkModule() error
dyld: Symbol not found: _rl_filename_completion_function
   Referenced from: /usr/local/lib/ruby/site_ruby/1.8/powerpc- 
darwin8.8.0/readline.bundle
   Expected in: flat namespace

Trace/BPT trap
############

### Steps to fix the above readline issue ###

~$  sudo rm -Rf /usr/local/lib/ruby
~$  mkdir tmp
~$  cd tmp
~/tmp$  wget "ftp://ftp.ruby-lang.org/pub/ruby/ruby-1.8.5.tar.gz"
~/tmp$  tar zxvf ruby-1.8.5.tar.gz
~/tmp$  cd ruby-1.8.5
~/tmp/ruby-1.8.5$  ./configure
~/tmp/ruby-1.8.5$  make
~/tmp/ruby-1.8.5$  sudo make install

~/tmp/ruby-1.8.5$  cd ext/readline
~/tmp/ruby-1.8.5/ext/readline$  make distclean
~/tmp/ruby-1.8.5/ext/readline$  ruby extconf.rb
~/tmp/ruby-1.8.5/ext/readline$  make
~/tmp/ruby-1.8.5/ext/readline$  sudo cp readline.bundle /usr/local/ 
lib/ruby/1.8/powerpc-darwin8.8.0/readline.bundle
~/tmp/ruby-1.8.5/ext/readline$  cd ~
~$  irb
irb(main):001:0>


Yes, I'm not sure if blowing away the /usr/local/lib/ruby dir was the  
best decision, but irb works now.
I hope this helps anyone else who experiences this same problem.

- Brian