ignore interface if it is Ddown (not 100% sure about vbox4.0 support; please test)

lib/vagrant/action/vm/network.rb

@@ -298,7 +298,7 @@ module Vagrant
 
           @env[:vm].driver.read_bridged_interfaces.each do |interface|
             that_netaddr = network_address(interface[:ip], interface[:netmask])
-            raise Errors::NetworkCollision if this_netaddr == that_netaddr
+            raise Errors::NetworkCollision if this_netaddr == that_netaddr && interface[:status] != "Down"
           end
         end
 

lib/vagrant/driver/virtualbox_4_0.rb

@@ -273,6 +273,8 @@ module Vagrant
               info[:ip] = $1.to_s
             elsif line =~ /^NetworkMask:\s+(.+?)$/
               info[:netmask] = $1.to_s
+            elsif line =~ /^Status:\s+(.+?)$/
+                info[:status] = $1.to_s
             end
           end
 

lib/vagrant/driver/virtualbox_4_1.rb

@@ -273,6 +273,8 @@ module Vagrant
               info[:ip] = $1.to_s
             elsif line =~ /^NetworkMask:\s+(.+?)$/
               info[:netmask] = $1.to_s
+            elsif line =~ /^Status:\s+(.+?)$/
+              info[:status] = $1.to_s
             end
           end