I just did the update and it work. When you rename the file, are you renaming it "update"? I know some that that tried to rename it "update.zip" but you don't have to add the ".zip" That is just the extension file name. When you rename the file ".zip" will automatically be included. It may not show when renaming because you may not have the show extension file name selected in your Windows OS option.
Windows 7 / Vista - Show File Extensions:
Start > Control Panel > Appearance & Personalization > Folder Options > View (tab) > Uncheck "Hide Extensions for known files types"
Windows XP - Show File Extensions:
Start > Control Panel > Folder Options > View (tab) > Uncheck "Hide Extensions for known files types"
This is the November 2010 OTA that added WiFi calling. Android 2.2. If you don't have this already then you would not likely be able to install the 2.3 OTA.