Index: test/unit/ajax.js
===================================================================
--- test/unit/ajax.js	(revision 6386)
+++ test/unit/ajax.js	(working copy)
@@ -874,6 +874,58 @@
 	})
 });
 
+test("jQuery.ajax - If-Modified-Since support", function() {
+	expect( 3 );
+
+	jQuery.ajaxSetup({ ifModified: true });
+
+	stop();
+
+	var url = "data/if_modified_since.php?ts=" + new Date();
+
+	jQuery.ajax({
+		url: url,
+		success: function(data, status) { 
+			equals(status, "success");
+			
+			jQuery.ajax({
+				url: url,
+				success: function(data, status) { 
+					equals(status, "notmodified");
+					ok(data == null, "response body should be empty")
+					start();
+				}
+			});
+		}
+	});
+});
+
+test("jQuery.ajax - Etag support", function() {
+	expect( 3 );
+
+	jQuery.ajaxSetup({ ifModified: true });
+
+	stop();
+
+	var url = "data/etag.php?ts=" + new Date();
+
+	jQuery.ajax({
+		url: url,
+		success: function(data, status) { 
+			equals(status, "success");
+			
+			jQuery.ajax({
+				url: url,
+				success: function(data, status) { 
+					equals(status, "notmodified");
+					ok(data == null, "response body should be empty")
+					start();
+				}
+			});
+		}
+	});
+});
+
 }
 
 //}
Index: src/ajax.js
===================================================================
--- src/ajax.js	(revision 6386)
+++ src/ajax.js	(working copy)
@@ -170,6 +170,7 @@
 
 	// Last-Modified header cache for next request
 	lastModified: {},
+	etag: {},
 
 	ajax: function( s ) {
 		// Extend the settings, but re-extend 's' so that it can be
@@ -298,10 +299,13 @@
 			if ( s.data )
 				xhr.setRequestHeader("Content-Type", s.contentType);
 
-			// Set the If-Modified-Since header, if ifModified mode.
-			if ( s.ifModified )
-				xhr.setRequestHeader("If-Modified-Since",
-					jQuery.lastModified[s.url] || "Thu, 01 Jan 1970 00:00:00 GMT" );
+				// Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
+				if ( s.ifModified ) {
+					if (jQuery.lastModified[s.url])
+						xhr.setRequestHeader("If-Modified-Since", jQuery.lastModified[s.url]);
+					if (jQuery.etag[s.url])
+						xhr.setRequestHeader("If-None-Match", jQuery.etag[s.url]);
+				}
 
 			// Set header so the called script knows that it's an XMLHttpRequest
 			xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
@@ -363,16 +367,7 @@
 				}
 
 				// Make sure that the request was successful or notmodified
-				if ( status == "success" ) {
-					// Cache Last-Modified header, if ifModified mode.
-					var modRes;
-					try {
-						modRes = xhr.getResponseHeader("Last-Modified");
-					} catch(e) {} // swallow exception thrown by FF if header is not available
-
-					if ( s.ifModified && modRes )
-						jQuery.lastModified[s.url] = modRes;
-
+				if ( status == "success" || status == "notmodified" ) {
 					// JSONP handles its own success callback
 					if ( !jsonp )
 						success();
@@ -467,13 +462,16 @@
 
 	// Determines if an XMLHttpRequest returns NotModified
 	httpNotModified: function( xhr, url ) {
-		try {
-			var xhrRes = xhr.getResponseHeader("Last-Modified");
+		var last_modified = xhr.getResponseHeader("Last-Modified");
+		var etag = xhr.getResponseHeader("Etag");
 
-			// Firefox always returns 200. check Last-Modified date
-			return xhr.status == 304 || xhrRes == jQuery.lastModified[url];
-		} catch(e){}
-		return false;
+		if (last_modified) 
+			jQuery.lastModified[url] = last_modified;
+
+		if (etag) 
+			jQuery.etag[url] = etag;
+
+		return xhr.status == 304;
 	},
 
 	httpData: function( xhr, type, s ) {