UPDATE: Blizzard officially announced Diablo 3 Saturday. Their Web site has a teaser trailer and gameplay footage, plus screenshots and information on two character classes, the returning Barbarian and new Witch Doctor.

Signs have popped up all over the Internet that Blizzard is prepared to announce Diablo 3 this Saturday at the Worldwide Invitational in Paris.

WoW Insider reports that a leaked schedule suggests a Diablo-related announcement, and DiabloII.net claims that industry insiders have confirmed that Blizzard will announce a new Diablo. Blizzard acquired the Web address diablo3.com earlier this year.

This information accompanies a piece of viral marketing on Blizzard’s homepage, which has been taken over by an evolving icy image. The ice has grown larger and opened up over the past three days, and two runic symbols have joined the original one.

The first symbol to appear comes from Frostmourne, the sword of the Lich King Arthas in Warcraft III. The World of Warcraft: Wrath of the Lich King expansion comes out in November. The second day’s rune is a Hel rune from the Diablo universe. The third is a Protoss marking from Starcraft. WoW Insider has an illustrated rundown.

UPDATE: Thursday’s rune comes from a canceled Blizzard game called Pax Imperium. Also new are two eyes behind the ice. Friday’s is a viking compass, emblematic of Lost Vikings.

The order in which these runes appeared matches Blizzard’s release history. Warcraft was the company’s first franchise, followed by Diablo and Starcraft.

Hidden in the page’s source code are links to six larger images, which, when put together, form an evil penguin.

I normally don’t like rumors, but this one is fun to follow. Check Blizzard.com tomorrow to see the next development in this marketing campaign.