...
#include <X11/Xlib.h>
#include <X11/Xutil.h>

/* After the standard includes, add the following: */
#include <hildon-widgets/hildon-program.h>
#include <gtk/gtkmain.h>
#include <gtk/gtk.h>

...

int main(int argc, char *argv[])
{
    Display    *dpy;            /* the X display to use */
    Window      win;            /* the actual X window */
    ...                         /* other variables we might use */
    /*
     * Add this to turn your program into a Hildon program:
     */
    HildonProgram *program;
    GdkWindow *gdkWindow;

    gtk_init(&argc, &argv);
    program = HILDON_PROGRAM(hildon_program_get_instance());
    g_set_application_name("Hello maemo!");

    /* Find the code that opens the Display and creates the X Window: */
    ...
    dpy = XOpenDisplay(NULL);
    ...
    win = XCreateWindow(dpy, ...);
    ...
    XMapWindow(dpy, win);

    /*
     * Now that the window has been mapped, we can turn it into a GDK window:
     */
    gdkWindow = gdk_window_foreign_new(win);
    ...

    /*
     * Add gtk_main_iteration() to the X11 event loop:
     */
    while (!quit)
    {
        XEvent event;

        XNextEvent(dpy, &event);

        ... /* handle X event as usual, then handle GTK events: */

        while (gtk_events_pending()) gtk_main_iteration();
    }

    /* Unreference the GDK window before destroying the X window: *
    
    g_object_unref(gdkWindow);
}