You can play sandbox mode now (offline, no opponent). It's not terribly exciting but you can at least play around with building units and such.

After installing the game (obviously) you need to:

1. Get the latest patch (Just run the Starcraft II launcher in the Starcraft II Beta Folder and it will auto-update).
2. Get maps from http://thepiratebay.org/torrent/5376948
3. Get the launcher from http://thepiratebay.org/torrent/5395...5BFIXED_ZIP%5D. Use it to load a map.