The blues have confirmed that patch 3.1 will be released tomorrow. Zarhym said as much in a post a few moments ago.
You can check out the full and final patch notes (Part I and Part II), and also take a look at WoW Insider's Guide to Patch 3.1 for all the latest information.
The realms will be down for extended maintenance tomorrow, and we're all hopeful that won't turn into some disaster like we've seen on previous patch days... but no one is betting any money on the success of the process right now.
Stay tuned to WoW Insider throughout the evening and tomorrow for the latest!