Starcraft 2 Arcade is currently available on the Patch 1.5.0 beta; the patch introduces a new Arcade button at the top left of the interface, this will now give access to all the custom games available for SC2. A Starcraft 2 button has also been introduced which leads you to the standard SC2 and Multiplayer. The Arcade is set to make sharing custom maps easier and allow greater feedback by allowing rating and reviewing of submitted maps by users. This has been done in conjuction with updates to the map editor to make it even easier for players to turn modders and start creating their own maps.
Blizzard have also imported a feature first seen in World of Warcraft with their streaming downloader for shorter update installation.
Just remember that this is still a beta patch which is available to download under your Battlenet account management page, there are likely still bugs to fix before it's put out for general release.