I have ported the under-mouse placement scheme to openbox3. Functionality is
exactly duplicated from openbox2. To enable, one must add
<placement><scheme>undermouse</scheme></placement> to the rc.xml configuration
file. This allows room for future placement schema and configuration options.

The patch follows against CVS. Files modified are config.h, config.c and place.c.

Index: openbox/config.c
===================================================================
RCS file: /cvs/cvsroot/openbox/openbox/config.c,v
retrieving revision 1.44
diff -c -r1.44 config.c
*** openbox/config.c	2003/09/04 02:36:05	1.44
--- openbox/config.c	2003/09/04 18:26:25
***************
*** 12,17 ****
--- 12,19 ----
  gboolean config_focus_last_on_desktop;
  guint    config_focus_delay;
  
+ gboolean config_place_umouse;
+ 
  char *config_theme;
  
  gchar *config_title_layout;
***************
*** 355,360 ****
--- 357,372 ----
          config_resist_edge = parse_int(doc, n);
  }
  
+ static void parse_placement(ObParseInst *i, xmlDocPtr doc, xmlNodePtr node, 
+                              void *d)
+ {
+ 	xmlNodePtr n;
+ 	node = node->xmlChildrenNode;
+ 	if ((n = parse_find_node("scheme", node)))
+             if (parse_contains("undermouse", doc, n))
+ 		    config_place_umouse = TRUE;		
+ }
+ 
  void config_startup(ObParseInst *i)
  {
      config_focus_new = TRUE;
***************
*** 364,369 ****
--- 376,385 ----
      config_focus_delay = 0;
  
      parse_register(i, "focus", parse_focus, NULL);
+     
+     config_place_umouse = FALSE;
+ 
+     parse_register(i, "placement", parse_placement, NULL);
  
      config_theme = NULL;
  
Index: openbox/config.h
===================================================================
RCS file: /cvs/cvsroot/openbox/openbox/config.h,v
retrieving revision 1.29
diff -c -r1.29 config.h
*** openbox/config.h	2003/09/03 08:12:07	1.29
--- openbox/config.h	2003/09/04 18:26:26
***************
*** 19,24 ****
--- 19,27 ----
  /*! Timeout for focusing windows on focus follows mouse, in microseconds */
  extern guint    config_focus_delay;
  
+ /*! Use undermouse placement */
+ extern gboolean config_place_umouse;
+ 
  /*! When true windows' contents are refreshed while they are resized; otherwise
    they are not updated until the resize is complete */
  extern gboolean config_redraw_resize;
Index: openbox/place.c
===================================================================
RCS file: /cvs/cvsroot/openbox/openbox/place.c,v
retrieving revision 1.1
diff -c -r1.1 place.c
*** openbox/place.c	2003/08/29 06:40:35	1.1
--- openbox/place.c	2003/09/04 18:26:26
***************
*** 3,8 ****
--- 3,10 ----
  #include "screen.h"
  #include "frame.h"
  
+ extern gboolean config_place_umouse;
+ 
  static Rect* pick_head(ObClient *c)
  {
      /* try direct parent first */
***************
*** 37,42 ****
--- 39,79 ----
      return NULL;
  }
  
+ /* Attempts to recreate the Ob2 Placement Under Mouse method. */
+ static gboolean place_umouse(ObClient *c, gint *x, gint *y)
+ {
+ 	int px, py, width, height;
+ 	unsigned int i = 0;
+ 	Rect *area;
+ 
+ 	screen_pointer_pos (&px, &py);
+ 
+ 	for (i = 0; i < screen_num_monitors; i++) {
+ 		area = screen_area_monitor (c->desktop, i);
+ 		if (RECT_CONTAINS(*area, px, py)) break;
+ 	}
+ 
+ 	if (c->frame != NULL) {
+ 		width = c->frame->area.width;
+ 		height = c->frame->area.height;
+ 	} else {
+ 		width = c->area.width;
+ 		height = c->area.height;
+ 	}
+ 
+ 	*x = px - width / 2;
+ 	*y = py - height / 2;
+ 
+ 	if (*x < area->x) *x = area->x;
+ 	if (*y < area->y) *y = area->y;
+ 	if (*x + width > area->x + area->width)
+ 		*x = area->x + area->width - width;
+ 	if (*y + height > area->y + area->height)
+ 		*y = area->y + area->height - height;
+ 
+ 	return TRUE;
+ }
+ 
  static gboolean place_random(ObClient *client, gint *x, gint *y)
  {
      int l, r, t, b;
***************
*** 130,135 ****
--- 167,174 ----
          return;
      if (place_dialog(client, x, y))
          return;
+     if (config_place_umouse && place_umouse(client, x, y))
+ 	return;
      if (place_random(client, x, y))
          return;
      g_assert_not_reached(); /* the last one better succeed */