A target is exclusive if the plaform doesn't match that of the parent.

lib/cocoapods/podfile.rb

@@ -10,9 +10,20 @@ module Pod
         options.each { |k, v| send("#{k}=", v) }
       end
 
-      def exclusive?
-        @exclusive
+      # A target is automatically `exclusive` if the `platform` does not match
+      # the parent's `platform`.
+      def exclusive
+        if @exclusive.nil?
+          if @platform.nil?
+            false
+          else
+            @parent.platform != @platform
+          end
+        else
+          @exclusive
+        end
       end
+      alias_method :exclusive?, :exclusive
 
       def link_with=(targets)
         @link_with = targets.is_a?(Array) ? targets : [targets]

spec/unit/podfile_spec.rb

@@ -89,6 +89,8 @@ describe "Pod::Podfile" do
 
         target :osx_target, :platform => :osx, :link_with => 'OSXTarget' do
           dependency 'ASIHTTPRequest'
+          target :nested_osx_target do
+          end
         end
 
         dependency 'ASIHTTPRequest'
@@ -166,6 +168,11 @@ describe "Pod::Podfile" do
       @podfile.target_definitions[:test].platform.should == :ios
       @podfile.target_definitions[:osx_target].platform.should == :osx
     end
+
+    it "autmatically marks a target as exclusive if the parent platform doesn't match" do
+      @podfile.target_definitions[:osx_target].should.be.exclusive
+      @podfile.target_definitions[:nested_osx_target].should.not.be.exclusive
+    end
   end
 
   describe "concerning validations" do