The FOP Ant task currently always renders the output.  The attached patch will
render only when the source file (.fo) is newer then the target (.pdf, etc.). 
There is also a "force" flag that can be used to always render.

This is the patch for org.apache.fop.tools.anttasks.Fop on the HEAD branch that
corresponds to the patch on the 0_20_2-maintain branch.  (The changes to the XML
documentation are the same for both branches.)

cvs diff -u src/java/org/apache/fop/tools/anttasks/Fop.java                    
 Index: src/java/org/apache/fop/tools/anttasks/Fop.java
===================================================================
RCS file: /home/cvspublic/xml-fop/src/java/org/apache/fop/tools/anttasks/Fop.java,v
retrieving revision 1.4
diff -u -u -r1.4 Fop.java
--- src/java/org/apache/fop/tools/anttasks/Fop.java     17 Jun 2003 16:35:58
-0000      1.4
+++ src/java/org/apache/fop/tools/anttasks/Fop.java     7 Jul 2003 20:44:17 -0000
@@ -94,6 +94,8 @@
  *      non-error messages</li>
  * <li>logFiles -> Controls whether the names of the files that are processed
  *      are logged or not</li>
+ * <li>force -> Recreate target files, even if they are newer than their
+ *      corresponding source files</li>
  * </ul>
  */
 public class Fop extends Task {
@@ -107,6 +109,7 @@
     private File userConfig;
     private int messageType = Project.MSG_VERBOSE;
     private boolean logFiles = true;
+    boolean force = false;         // Ignore modification dates if true
 
     /**
      * Sets the filename for the userconfig.xml.
@@ -270,6 +273,20 @@
     }
 
     /**
+     * Sets force (ignore mod dates) flag 
+     */
+    public void setForce(boolean aBoolean) {
+        this.force = aBoolean;
+    }
+
+    /**
+     * Gets force (ignore mod dates) flag
+     */
+    public boolean getForce() {
+        return this.force;
+    }
+
+    /**
      * @see org.apache.tools.ant.Task#execute()
      */
     public void execute() throws BuildException {
@@ -410,8 +427,13 @@
                 if (task.getOutdir() != null) {
                     outf = new File(task.getOutdir(), outf.getName());
                 }
-                render(task.getFofile(), outf, rint);
-                actioncount++;
+                // Only render if "force" flag is set OR output file doesn't
exist OR
+                // output file is older than input file
+                if ( task.force || !outf.exists() ||
+                    (task.getFofile().lastModified() > outf.lastModified() ) ) {
+                    render(task.getFofile(), outf, rint);
+                    actioncount++;
+                }
             }
         }
 
@@ -449,14 +471,17 @@
                     task.log("Error setting base URL", Project.MSG_DEBUG);
                 }
 
-                render(f, outf, rint);
-                actioncount++;
+                // Only render if "force" flag is set OR output file doesn't
exist OR
+                // output file is older than input file
+                if ( task.force || !outf.exists() || (f.lastModified() >
outf.lastModified() ) ) {
+                    render(f, outf, rint);
+                    actioncount++;
+                }
             }
         }
 
         if (actioncount == 0) {
-            task.log("No files processed. No files were selected by the filesets "
-                + "and no fofile was set." , Project.MSG_WARN);
+            task.log("Nothing to do." , Project.MSG_WARN);
         }
     }