At work today we got bit by a DTD URL change. Thanks Oracle.
One of our web applications would not deploy, giving a java.io.FileNotFoundException for a DTD at this url:
http://www.sun.com/software/appserver/dtds/sun-ejb-jar_3_1-0.dtd.
A little bit of Google sleuthing turned up a working URL for this DTD at this url:
http://www.sun.com/software/dtd/appserver/sun-ejb-jar_3_1-0.dtd
Update
It appears that a lot of DTDs are changed. There is a list of current DTDs here: http://www.sun.com/software/dtd/appserver/
You have saved my life!
Today I had the same problem with
http://www.sun.com/software/appserver/dtds/sun-web-app_3_0-0.dtd
The working link, as you suggested, is
http://www.sun.com/software/dtd/appserver/sun-web-app_3_0-0.dtd
Thank you, William!