Are you ready for Patch 4.0.1? We hope so, because it might be here tomorrow.
If our buddies over at MMO-Champion are right (read: they usually are about this stuff), then that means that World of Warcraft's Patch 4.0.1 will be hitting live servers tomorrow. All the usual signs are there: the current arena season is ending, a long maintenance is scheduled for tomorrow, there are tons of official previews; the only thing we don't have is an official confirmation (read: still file this under rumors and possibility).
That said, when 4.0.1 does hit, there are a lot of pretty huge changes coming, both on a general scale (glyph changes, all talent trees dramatically changed, badge conversions, gem color changes, etc.) to a class-specific scale (Death Knight rune regeneration changed, Hunters no longer need ammo or mana, etc.). If you haven't done so already, it is highly recommended that you take a look at the changes to your class and plan accordingly. Also, if arrows are your money maker, get rid of them now! Tomorrow they (probably) turn into vendor loot. In addition, if you've been saving up honor for gem purchases so you can make a killing off everyone that needs to re-gem, buy them now - the vendor will be going the way of Captain Placeholder with the introduction of 4.0.1.
If you're interested in finding out more about the more serious class mechanic changes, check out this post from Bashiok on the official World of Warcraft forums. Good luck in your preparation!
UPDATE: 100% confirmed, via Bashiok in this post. Enjoy!