WordPress 3.5 AKA “Elvin” is out now. 3.5 features a wonderful new look for the media uploader, a clean new default theme and more. However, I DON’T recommend getting trigger-happy with the update button just yet. I’m hearing of a few plugin conflicts and other issues so it would be wise to test out your site with 3.5 on a sandbox installation i.e. a replica of you site either on a spare domain name, a subfolder, or a local WP installation on your computer, in order to see how things run. At a bare minimum make sure you have a FULL (database and files) backup of your site before you upgrade.
You may also find these useful