Update 2 - Removed the news about Diablo 3 forums.
Update - The splash has been updated with details on the latest rune.
Official site teaser/splash screen - Day 5
This is the last day before the WWI, the eyes in the middle of the screen really start to look like the Lich King's ... However the creature artwork has been fully uploaded (we're not sure if it's an error or not) and we now have the full picture available.
The splash screen
This is the most obvious part of the teaser, a huge picture of a block of ice slowly cracking to reveal what seems to be a face. Each day a rune was added to the ice.
- Day 1 - The first rune looks a lot like the one found on the Lich King swords in many official artworks.
- Day 2 - The 2nd rune is the Hel Rune from Diablo 2
- Day 3 - The 3rd rune is the Protoss Symbol found on the Starcraft 2 official site
- Day 4 - The 4th rune hasn't been identified yet, and what looks like the eyes of the lich king appeared in the center of the ice crack.
- Day 5 - A new rune has been added and looks like a viking compass. The eyes started to glow, the creature artwork from the CSS files is now complete. The runes also started to glow, you can compare these screenshots : Screenshot #1 / Screenshot #2
The "hidden" artwork
If you check the CSS File of the page, you'll notice a few commented lines pointing to image links.
/*.tundra {background:url(_images/04.jpg) no-repeat 0 0} */
/*.harbinger {background:url(_images/08.jpg) no-repeat 0 0} */
/*.cold {background:url(_images/16.jpg) no-repeat 0 0} */
/*.cavern {background:url(_images/15.jpg) no-repeat 0 0} */
/*.icy {background:url(_images/23.jpg) no-repeat 0 0} */
/*.evilpenguin {background:url(_images/42.jpg) no-repeat 0 0} */
There was a few theories about these pictures, people tried to replace the number of each pictures with letter to see if it would form "Diablo" in the end but it didn't work. Obviously there are many links between "icy", "tundra", "cavern", "cold", and Wrath of the Lich King / Diablo 2 and we can't confirm anything based on these names.
However, now the link with the Numbers from Lost is definitely proven.
Conclusion
Still Diablo 3, and you won't get any more proof until it's announced tomorrow.