Hmm so after using this filter for a few days it now appears that whilst it catches all email already in your inbox it does not apply the same filter to newly received messages. This makes this entire tip pretty useless... I'll keep looking for a solution to this but for the time being I don't recommend you use the filter described below. If you have any tips please leave a comment.

If you are like me you may be using Google Apps (or GMail) in combination with an IMAP enabled email client such as Mail or Outlook. Recently I noticed that I was missing some emails which were not caught by any of my filters: in a nutshell, I filter emails for each domain I want to receive email for by applying a label which then shows up as an IMAP folder in OSX Mail. However some senders, particularly bulk emails such as newsletters, specify the 'to' address as either undisclosed or even as the sender address rather than the actual recipient's email address (in this case that'd be mine) which in turn causes the emails to sit in my inbox amongst thousands of other emails where I may miss them.
I thought it would be easy to simply create a new label for unread emails in GMail but unfortunately the label 'unread' is reserved by Google (it did not use to be). You can of course also run a search for unread emails but that does not give you the ability to automatically label the matching messages. To workaround this you can do the following:
1) create a new label such as '_unread'
2) create a new filter with the criteria of 'label:unread' in the 'Has the words:' field
3) Test the filter: all your unread emails should show up
4) click 'Next Step' and ignore the warning
5) apply the label you created in step 1

You should now have a new folder in your email client that shows you all unread emails in GMail. One thing to note is that your local folder may also show any corresponding read messages which are tied to the same conversation as an unread message.