Quick Summary: The 5th day Rune refers to the Lost Vikings franchise, as possibly do the 'Lost' numbers. Two eyes have been reinforced at the centre of the image, and given the colour and glow effect they most resemble the eyes of the Lich King Arthas. The Tiled image has been revealed to show an 'Evil Penguin', and the numbers on the Tiled Images do match up with the 'Lost' number sequence. My gut instinct is it's a 'Wrath of the Lich King' announcement of some kind. Nothing we've seen so far provides any concrete evidence that Diablo 3 is going to be announced. Still, we live in hope.
UPDATE: Okay, I'm being an idiot and not really thinking things through, it is the Lost numbers, but those numbers don't spell out any 'TYRAEL IS GONE'-style phrase from Yesterday's theory due to the lack of an 'e'.
Morning all, lets get down to business. Todays Splash Screen is ice5.jpg:
Damn. Well they are definitely a pair of eyes, I'm not sure which eyes they match the best though (Arthas/The Lich King?), more on that later. Ice5.jpg has the following rune:
A Viking Compass Perhaps (hat tip Flowz0r in comments).
Implying it's to do with the Lost Vikings franchise, and the 4th rune is another Franchise. One of the theories floating around is that it relates to Pax Imperia: Eminent Domain because that was a space-based RTS which matched the planetary motif. However Blizzard only worked on that game, not the original, and sold the rights to THQ before it was released (the game was eventually developed by another software house). Unless the original authors are now working for Blizzard I don't find that a credible theory, so the meaning of the 4th Rune is effectively still up for grabs.
The location of the rune gives credence to all those guys and gals who wanted the rune arrangement to be a pentagram... it is one, but only very loosly. All the other Runes are much clearer now, and we can see that the one from the first day is in fact the top-most rune from Frostmourne.
Okay, missed that. There are two words today, from the master CSS files: 'harbinger' and 'evilpenguin'; and two associated 'hidden image tiles: 08.jpg and 42.jpg. Yay for theories: 08.jpg does fit the Lost number sequence... and 42 does also. Neither of these number+word combinations would provide the necessary 'E' for spelling out Tyrael however, never mind any message for him, so that specific conjecture theory is out the window too.
As for who the 'Harbinger' might be: "You are the harbinger of our return, Diablo!" (the words of Mephisto, Video is the Diablo 2 Act IV Cinematic)
Finally, the hidden image's tiles, constructed together look like the following:
Yep, that's one evil looking penguin. First Question... who's Artists' Signature is that?
Second Question: Whose are those Eyes? My Gut instinct is the Lich King, making this an announcement concerning Wrath of the Lich King, but is the pentagram Rune formation significant?
Third Question: What does the last unknown Rune, the 4th Rune which looks like a planetary system, represent?
EDIT1: Thanks to Anonymous in comments, here is an image overlaying a Diablo drawn by one of the Suns of the Storm and the current status of the splash screen image done by a member of Diablo.leforum.eu:
As always with overlays I'm quite sceptical, but it is well worth mentioning.