Skin Previewer was not working right for me. It would put two 'skin=' strings in the URL, e.g. https://mail.domain.com/?skin=beach&skin=bones. It would only display in the default skin. I changed the skinpreviewer.js file to strip out the default skin, then undeployed and redeployed.

Code:
[zimbra@mail com_zimbra_skinpreviewer.orig]$ diff -u skinpreviewer.js ../com_zimbra_skinpreviewer/skinpreviewer.js
--- skinpreviewer.js	2010-02-03 00:49:22.000000000 -0700
+++ ../com_zimbra_skinpreviewer/skinpreviewer.js	2010-02-21 18:47:39.000000000 -0700
@@ -102,7 +102,8 @@
 
 com_zimbra_skinpreviewer.prototype._previewSelectedSkins =
 function() {
-	var url = AjxUtil.formatUrl({});
+	var fullurl = AjxUtil.formatUrl({});
+	var url = fullurl.split("?")[0];
 	var ch = "";
 	if (url.indexOf("?") > 0) {
 		ch = "&skin=";