It struck me later that you're probably designing for the use case where the client is gone and really isn't coming back. In that case, perhaps a banner at the top of the OmniFocus window would be better than a modal dialog. I'm thinking of something like what you see at the top of
http://stackoverflow.com when there are new messages for you. It could sit there until you dismiss it or act on it. The problem with a modal is that it interrupts my flow and makes me feel like I must do something about it now.