On OS X Panther:
 > ruby -v
ruby 1.8.1 (2003-12-25) [powerpc-darwin]

 > irb
irb(main):001:0> require 'socket'
=> true
irb(main):002:0> Socket::getaddrinfo('192.168.1.100', 8081, 
Socket::AF_UNSPEC, Socket::SOCK_STREAM,  0,  Socket::AI_PASSIVE)
SocketError: getnameinfo: nodename nor servname provided, or not known
         from (irb):2:in `getaddrinfo'
         from (irb):2

On Linux:
 > ruby -v
ruby 1.8.0 (2003-08-04) [i686-linux]

 > irb
irb(main):001:0> require 'socket'
=> true
irb(main):002:0> Socket::getaddrinfo('192.168.1.100', 8081, 
Socket::AF_UNSPEC, Socket::SOCK_STREAM,  0,  Socket::AI_PASSIVE)
=> [["AF_INET", 8081, "192.168.1.100", "192.168.1.100", 2, 1, 6]]

This line comes from Webrick util.rb file (line 52), which does not 
work on OS X right now because of that error.

-rich