I had annt interesting discussion with a colleague of mine last evening regarding how to set up a SharePoint Designer Workflow so that in this case, only sends an email when a specific field changes. After a short refresh of my memory with SD i have to say that I did not see how that could be done without having a Visual Studio Workflow.
But then I got a link from my colleague to an article and a screecast made by Stephen Howard that shows one way to accomplish it. Check it out at: http://office.microsoft.com/en-us/sharepointdesigner/HA102564191033.aspx