Added the new .offset() method, directly imported from Dimensions (heavily worked over by both Brandon and John).

Original source:
http://brandonaaron.net/jquery/issues/dimensions/new_offset/jquery.offset.js

Author: jeresig

Makefile

@@ -12,7 +12,8 @@ BASE_FILES = ${SRC_DIR}/core.js\
 	${SRC_DIR}/selector.js\
 	${SRC_DIR}/event.js\
 	${SRC_DIR}/ajax.js\
-	${SRC_DIR}/fx.js
+	${SRC_DIR}/fx.js\
+	${SRC_DIR}/offset.js
 
 PLUGINS = ${PLUG_DIR}/button/*\
 	${PLUG_DIR}/center/*\

build.xml

@@ -48,8 +48,9 @@
             <fileset dir="${SRC_DIR}" includes="core.js" />
             <fileset dir="${SRC_DIR}" includes="selector.js" />
             <fileset dir="${SRC_DIR}" includes="event.js" />
-            <fileset dir="${SRC_DIR}" includes="fx.js" />
             <fileset dir="${SRC_DIR}" includes="ajax.js" />
+            <fileset dir="${SRC_DIR}" includes="fx.js" />
+            <fileset dir="${SRC_DIR}" includes="offset.js" />
             <fileset dir="${PLUGIN_DIR}" includes="${PLUGINS}" />
             <fileset dir="${SRC_DIR}" includes="outro.js" />
         </concat>

src/offset.js

@@ -0,0 +1,88 @@
+jQuery.fn.offset = function() {
+	var left = 0, top = 0, elem = this[0], results;
+	
+	if ( elem ) with ( jQuery.browser ) {
+		var	absolute	= jQuery.css(elem, "position") == "absolute", 
+		    	parent		= elem.parentNode, 
+		    	offsetParent	= elem.offsetParent, 
+		    	doc		= elem.ownerDocument,
+			safari2		= safari && !absolute && parseInt(version) < 522;
+	
+		// Use getBoundingClientRect if available
+		if ( elem.getBoundingClientRect ) {
+			box = elem.getBoundingClientRect();
+		
+			// Add the document scroll offsets
+			add(
+				box.left + Math.max(doc.documentElement.scrollLeft, doc.body.scrollLeft),
+				box.top  + Math.max(doc.documentElement.scrollTop,  doc.body.scrollTop)
+			);
+		
+			// IE adds the HTML element's border, by default it is medium which is 2px
+			// IE 6 and IE 7 quirks mode the border width is overwritable by the following css html { border: 0; }
+			// IE 7 standards mode, the border is always 2px
+			if ( msie ) {
+				var border = jQuery("html").css("borderWidth");
+				border = (border == "medium" || jQuery.boxModel && parseInt(version) >= 7) && 2 || border;
+				add( -border, -border );
+			}
+	
+		// Otherwise loop through the offsetParents and parentNodes
+		} else {
+		
+			// Initial element offsets
+			add( elem.offsetLeft, elem.offsetTop );
+		
+			// Get parent offsets
+			while ( offsetParent ) {
+				// Add offsetParent offsets
+				add( offsetParent.offsetLeft, offsetParent.offsetTop );
+			
+				// Mozilla and Safari > 2 does not include the border on offset parents
+				// However Mozilla adds the border for table cells
+				if ( mozilla && /^t[d|h]$/i.test(parent.tagName) || !safari2 )
+					border( offsetParent );
+				
+				// Safari <= 2 doubles body offsets with an absolutely positioned element or parent
+				if ( safari2 && !absolute && jQuery.css(offsetParent, "position") == "absolute" )
+					absolute = true;
+			
+				// Get next offsetParent
+				offsetParent = offsetParent.offsetParent;
+			}
+		
+			// Get parent scroll offsets
+			while ( parent.tagName && /^body|html$/i.test(parent.tagName) ) {
+				// Work around opera inline/table scrollLeft/Top bug
+				if ( /^inline|table-row.*$/i.test(jQuery.css(parent, "display")) )
+					// Subtract parent scroll offsets
+					add( -parent.scrollLeft, -parent.scrollTop );
+			
+				// Mozilla does not add the border for a parent that has overflow != visible
+				if ( mozilla && jQuery.css(parent, "overflow") != "visible" )
+					border( parent );
+			
+				// Get next parent
+				parent = parent.parentNode;
+			}
+		
+			// Safari doubles body offsets with an absolutely positioned element or parent
+			if ( safari && absolute )
+				add( -doc.body.offsetLeft, -doc.body.offsetTop );
+		}
+
+		// Return an object with top and left properties
+		results = { top: top, left: left };
+	}
+
+	return results;
+
+	function border(elem) {
+		add( jQuery.css(elem, "borderLeftWidth"), jQuery.css(elem, "borderTopWidth") );
+	}
+
+	function add(l, t) {
+		left += parseInt(l) || 0;
+		top += parseInt(t) || 0;
+	}
+};