Was just doing some surfing and ran across this about "Service Flags"-
"The Service flag was first displayed in the front windows of homes during World War I to signify a son or husband serving in the Armed Forces. The flag quickly became known as the "son in service flag" with each blue star indicating one family member."
On the web page www.armywives.com, if you scroll down a bit to the Service Flags entry you'll see that there is a link to learn more about the history of these flags, plus you can buy one... but there is also a link for a "printable flag". If you have a color copier it comes out real nice.