DRb stores the current client in Thread.current['DRb']['client'], but 
there is no 'friendly' method to access it like 
Thread.current['DRb']['server']'s #current_server.

I wrote the following patch to add #current_client:

Index: lib/drb/drb.rb
===================================================================
RCS file: /src/ruby/lib/drb/drb.rb,v
retrieving revision 1.21
diff -u -r1.21 drb.rb
--- lib/drb/drb.rb      28 Jul 2004 09:05:07 -0000      1.21
+++ lib/drb/drb.rb      10 Dec 2004 22:19:35 -0000
@@ -401,6 +401,11 @@
    # See #current_server.
    class DRbServerNotFound < DRbError; end

+  # Error raised by the DRb module when an attempt is made to refer to
+  # the context's current drb client but the context does not have one.
+  # See #current_client.
+  class DRbClientNotFound < DRbError; end
+
    # Error raised by the DRbProtocol module when it cannot find any
    # protocol implementation support the scheme specified in a URI.
    class DRbBadURI < DRbError; end
@@ -1529,6 +1534,23 @@
      return server
    end
    module_function :current_server
+
+  # Get the 'current' client.
+  #
+  # In the context of execution taking place within the main
+  # thread of a dRuby server (typically, as a result of a remote
+  # call on the server or one of its objects), the current
+  # client is that client.
+  #
+  # If the above rule fails to find a client, a DRbClientNotFound
+  # error is raised.
+  def current_client
+      drb = Thread.current['DRb']
+      client = (drb && drb['client']) ? drb['client'] : nil
+      raise DRbClientNotFound unless client
+      return client
+  end
+  module_function :current_client

    # Stop the local dRuby server.
    #