Plumiferos report:

The new Material "LightGroups" only worked with lamps in visible layers.
Now also lamps from the group that are not visible are included for
rendering, ensuring that a lightgroup always works on that material,
disregarding layer settings (unless lamp is type 'layer lamp').

source/blender/blenkernel/intern/material.c

@@ -102,7 +102,6 @@ void free_material(Material *ma)
 
 void init_material(Material *ma)
 {
-	ma->lay= 1;
 	ma->r= ma->g= ma->b= ma->ref= 0.8;
 	ma->specr= ma->specg= ma->specb= 1.0;
 	ma->mirr= ma->mirg= ma->mirb= 1.0;
@@ -658,9 +657,12 @@ void init_render_materials(int osa, float *amb)
 	Material *ma;
 	
 	/* two steps, first initialize, then or the flags for layers */
-	for(ma= G.main->mat.first; ma; ma= ma->id.next)
+	for(ma= G.main->mat.first; ma; ma= ma->id.next) {
+		/* is_used flag comes back in convertblender.c */
+		ma->flag &= ~MA_IS_USED;
 		if(ma->id.us) 
 			init_render_material(ma, osa, amb);
+	}
 }
 
 /* only needed for nodes now */

source/blender/makesdna/DNA_material_types.h

@@ -54,7 +54,7 @@ struct bNodeTree;
 typedef struct Material {
 	ID id;
 	
-	short colormodel, lay;		/* lay: for dynamics (old engine, until 2.04) */
+	short colormodel, flag;	
 	/* note, keep this below synced with render_types.h */
 	float r, g, b;
 	float specr, specg, specb;
@@ -129,6 +129,10 @@ typedef struct Material {
 #define MA_YUV			2
 #define MA_HSV			3
 
+/* flag */
+		/* for render */
+#define MA_IS_USED		1
+
 /* mode (is int) */
 #define MA_TRACEBLE		1
 #define MA_SHADOW		2

source/blender/render/intern/source/convertblender.c

@@ -1007,6 +1007,9 @@ static Material *give_render_material(Render *re, Object *ob, int nr)
 	
 	if(re->r.mode & R_SPEED) ma->texco |= NEED_UV;
 	
+	/* for light groups */
+	ma->flag |= MA_IS_USED;
+	
 	return ma;
 }
 
@@ -2150,7 +2153,7 @@ static void area_lamp_vectors(LampRen *lar)
 }
 
 /* If lar takes more lamp data, the decoupling will be better. */
-static void add_render_lamp(Render *re, Object *ob)
+static LampRen *add_render_lamp(Render *re, Object *ob)
 {
 	Lamp *la= ob->data;
 	LampRen *lar;
@@ -2161,7 +2164,7 @@ static void add_render_lamp(Render *re, Object *ob)
 	/* prevent only shadow from rendering light */
 	if(la->mode & LA_ONLYSHADOW)
 		if((re->r.mode & R_SHADOW)==0)
-			return;
+			return NULL;
 	
 	go= MEM_callocN(sizeof(GroupObject), "groupobject");
 	BLI_addtail(&re->lights, go);
@@ -2347,6 +2350,7 @@ static void add_render_lamp(Render *re, Object *ob)
 				lar->mode &= ~LA_SHAD_RAY;
 		}
 	}
+	return lar;
 }
 
 /* ------------------------------------------------------------------------- */
@@ -3092,11 +3096,15 @@ static void set_material_lightgroups(Render *re)
 		if(ma->group) {
 			for(go= ma->group->gobject.first; go; go= go->next) {
 				go->lampren= NULL;
-				for(gol= re->lights.first; gol; gol= gol->next) {
-					if(gol->ob==go->ob) {
-						go->lampren= gol->lampren;
-						break;
+				if(go->ob && go->ob->type==OB_LAMP) {
+					for(gol= re->lights.first; gol; gol= gol->next) {
+						if(gol->ob==go->ob) {
+							go->lampren= gol->lampren;
+							break;
+						}
 					}
+					if(go->lampren==NULL) 
+						go->lampren= add_render_lamp(re, go->ob);
 				}
 			}
 		}

source/blender/src/buttons_shading.c

@@ -2524,12 +2524,6 @@ void do_matbuts(unsigned short event)
 			scrarea_queue_winredraw(curarea);
 		}
 		break;
-	case B_MATLAY:
-		if(ma && ma->lay==0) {
-			ma->lay= 1;
-			scrarea_queue_winredraw(curarea);
-		}
-		break;
 	case B_MATZTRANSP:
 		if(ma) {
 			ma->mode &= ~MA_RAYTRANSP;