Starting with patch 1.2, you'll be able to preview and test most, if not all, StarCraft II patches before they go public. Blizzard has announced plans to set up a test server for this latest patch and future updates from this point forward, which is something the studio has already been doing with big updates for World of Warcraft. The primary purpose of this test, as explained on the SCII blog, is to help Blizzard identify SCII patch bugs and "ensure that when a patch is released, it's of the highest quality possible."
Blizzard has a FAQ in place that describes what you need to do in order to get into this "PTR." Essentially, all you need is an SCII account that was created before November 11 and a working knowledge of how to fiddle with the RTS' install directory.
From the FAQ:
When the PTR is available, a retail patch will add a new executable file named "StarCraft II Public Test.exe" to the StarCraft II game directory. To access the PTR, go to the StarCraft II game directory and run the "StarCraft II Public Text.exe" executable.
A small patch will be downloaded to install the PTR client the first time you run it. Eligible accounts will already be enabled for the PTR so there's no sign-up needed for this phase.
== TEASER ==Patch 1.2 is a startlingly huge update, which might explain why Blizzard has chosen it to be first patch to be tested by fans before full release. It adds Battle.net chat, a massive amount of map editor tweaks and fixes, a few race balances, and tons of general bug fixes. Shack has the full patch list, which'll take you about 10 minutes to read from beginning to end if you're the kinda guy who really gets into this sort of thing.
As Blizzard notes, PTRs are tests, so do expect some shutdowns or instances of poor performance. If you find yourself hitting one too many hitches, just switch over the actual game. You'll be able to do that at any point, even if you install the PTR client.