Works for me.

Makefile

@@ -0,0 +1,4 @@
+CFLAGS = -Wall
+LDLIBS = -lX11 -lXss
+
+all: xss

xss.c

@@ -0,0 +1,98 @@
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <X11/Xlib.h>
+#include <X11/Xatom.h>
+#include <X11/extensions/scrnsaver.h>
+
+int child = 0;
+
+void
+sigchld(int signum)
+{
+  /* Reap zombies */
+  int   status;
+
+  (void)wait(&status);
+  child = 0;
+}
+
+int
+main(int argc, char *argv[])
+{
+  Display *display = NULL;
+  char    *errtxt  = "bummer dude.";
+  Pixmap   blank;
+  int      screen;
+  int      ss_event, ss_error;
+  Window   root;
+
+
+  if (argc < 2) {
+    fprintf(stderr, "Usage: %s PROGRAM [ARGUMENT ...]\n", argv[0]);
+    exit(1);
+  }
+
+  do {
+    display = XOpenDisplay(NULL);
+    if (! display) {
+      errtxt = "Cannot open display";
+      break;
+    }
+
+    root = DefaultRootWindow(display);
+
+    if (! XScreenSaverQueryExtension(display, &ss_event, &ss_error)) {
+      errtxt = "X server does not support MIT-SCREEN-SAVER extension.";
+      break;
+    }
+
+    XScreenSaverSelectInput(display, root, ScreenSaverNotifyMask);
+
+    signal(SIGCHLD, sigchld);
+
+    while (1) {
+      XEvent event;
+
+      if (XNextEvent(display, &event)) {
+        errtxt = NULL;
+        break;
+      }
+
+      if (event.type == ss_event) {
+        XScreenSaverNotifyEvent *sevent;
+
+        sevent = (XScreenSaverNotifyEvent *)&event;
+        if (ScreenSaverOn == sevent->state) {
+          if (child) {
+            /* Don't do anything, we still have a child */
+          } else {
+            child = fork();
+            if (0 == child) {
+              (void)execvp(argv[1], argv + 1);
+              perror("exec");
+              exit(1);
+            }
+          }
+        }
+      }
+    }
+  } while (0);
+
+  if (display) {
+    (void)XScreenSaverUnregister(display, screen);
+    (void)XFreePixmap(display, blank);
+    (void)XCloseDisplay(display);
+  }
+
+  if (errtxt) {
+    fprintf(stderr, "Error: %s\n", errtxt);
+    return 1;
+  }
+
+  return 0;
+}