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.

cvs diff -u src/documentation/content/xdocs/anttask.xml
src/org/apache/fop/tools/anttasks/Fop.java 
Index: src/documentation/content/xdocs/anttask.xml
===================================================================
RCS file: /home/cvspublic/xml-fop/src/documentation/content/xdocs/anttask.xml,v
retrieving revision 1.3.2.1
diff -u -u -r1.3.2.1 anttask.xml
--- src/documentation/content/xdocs/anttask.xml 19 May 2003 11:20:27 -0000     
1.3.2.1
+++ src/documentation/content/xdocs/anttask.xml 7 Jul 2003 20:38:12 -0000
@@ -90,7 +90,12 @@
                        (<code>true</code>) or not (<code>false</code>)</td> 
        <td>No, default is <code>true</code></td> 
       </tr> 
-     </table>
+      <tr> 
+       <td>force</td> 
+       <td>Recreate target files, even if they are newer than their
corresponding source files.</td> 
+       <td>No, default is <code>true</code></td> 
+      </tr> 
+    </table>
         <p/>
         <table><caption>Parameters specified as nested elements</caption>
       <tr> 
@@ -104,7 +109,7 @@
                        are used to specify multiple XSL-FO files to be
rendered.</td> 
        <td>Yes, if no fofile attribute is supplied</td> 
       </tr> 
-         </table>
+        </table>
     </section>
     <section>
     <title>Examples</title>
Index: src/org/apache/fop/tools/anttasks/Fop.java
===================================================================
RCS file: /home/cvspublic/xml-fop/src/org/apache/fop/tools/anttasks/Attic/Fop.java,v
retrieving revision 1.14.2.8
diff -u -u -r1.14.2.8 Fop.java
--- src/org/apache/fop/tools/anttasks/Fop.java  25 Feb 2003 15:19:58 -0000     
1.14.2.8
+++ src/org/apache/fop/tools/anttasks/Fop.java  7 Jul 2003 20:38:12 -0000
@@ -92,6 +92,7 @@
  * <li>userconfig -> file with user configuration (same as the "-c" command
line option)</li>
  * <li>messagelevel -> (error | warn | info | verbose | debug) level to output
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 {
@@ -105,6 +106,7 @@
     File userConfig;
     int messageType = Project.MSG_VERBOSE;
     boolean logFiles = true;
+    boolean force = true;         // Ignore modification dates if true
 
     /**
      * Sets the input file
@@ -238,6 +240,20 @@
     }
 
     /**
+     * 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;
+    }
+
+    /**
      * Starts execution of this task
      */
     public void execute() throws BuildException {
@@ -369,8 +385,13 @@
                 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++;
+                }
             }
         }
 
@@ -413,15 +434,17 @@
                     task.log("Error setting base directory: " + e,
Project.MSG_ERR);
                 }
 
-                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);
         }
     }