I know that that ESRI will be putting something like this out soon, but much better..... However, I had a project that I needed hundreds of popup.xml files, so I wrote a little python script to do it. I'm not aware of another utility out there to do this, but then again maybe there is. I'm sure it isn't perfect but it does work for me. Direction can be found within the zip file.
There are two python scripts, one that allows you to point it to a feature class and generate a basic popup.xml file for the feature class. There then is another that you can point to a FGDB and generate popup.xml file for every feature class in it. In my experience it takes about 5 minutes to generate 50 popup.xml files. That's much faster then I can do it by hand.
