The Diablo 3 camp, for its part, generally considers the image to be that of the worldstone -- the worldstone is actually red instead of blue, but there's always a possibility the color could have changed due to the events of Diablo 2. You'll remember that at the end of Diablo 2: Lord of Destruction, the archangel Tyrael was forced to destroy the worldstone after Baal defiled it. Even he didn't know what would happen once it was broken. The WoTLK Beta camp, on the other hand, considers it to be just plain old ice - the file name of the splash images have been ice1.jpg and ice2.jpg, after all.
The images behind the ice, which can barely be seen, look to be some sort of runes. Some people claim that those runes are similar to ones on Frostmourne, while others claim they are runes from Diablo 2's rune system -- The lower right one seems to be Shael, while the upper right is Hel, according to them. The Diablo 3 crowd also sees quite a few other things under the Ice. For example, Evamortte of the US-Tichondrius server sees a number 3 carved into the ice on the far right side of the image.
We've also recieved quite a few other theories from tipsters. Tipster Crak911 believes that the ice is hiding a picture of Diablo, and sent us an outline of what he sees under the ice to prove it. Fellow WoW Insider blogger Natalie Mootz isn't so sure. She imposed Crak911's drawing over the splash page itself, then imposed a Diablo 3 box mockup over both the drawing and the splash page. She's not convinced that Crak's drawing quite measures up, and neither am I, but the speculation was still fun, and the outline does seem to follow the cracks in the ice well enough. Another anonymous tipster believes that the rune circle seen in the lower left circle looks similar to the circle of the Ancients on Arreat Summit in Diablo 2: Lord of Destruction, as well as a circular representation of the levels of hell.
However, the most compelling evidence for the Diablo 3 camp comes from a tipster calling himself Blurred:
He dug into the workings of WoW's website to give us some ideas on what could be going on. He notes first that the folder name with the splash screen images on Blizzard's site is called 232309. If you consider that W is the 23rd letter of the alphabet, and I the 9th, that would spell WWI -- Worldwide Invitational. Likewise, also located in that folder is a strange image that is obviously part of a larger one, named 04.JPG -- which might translate to the D in Diablo. The master CSS file for June 23rd was also appended with the name "tundra" in CSS.
To top that off, another secret image was uploaded today, this one being found by diablofans.com, named 15.jpg -- which could translate to the O in Diablo. It's very likely that 15.jpg and 04.jpg are part of the same picture -- which appears to be that of some sort of purple monster. In addition, the master CSS file for today was appended with the name "cavern." Diablofans.com speculates that the names in the CSS could refer to zones in Diablo 2: Lord of Destruction, namely the Frozen Tundra and the Drifter's Cavern. Of course, the WOTLK camp could argue back that these could just as easily refer to features that a frozen wasteland like Northrend could have.
With that last bit of information, I feel pretty secure in saying that we really can expect some Diablo 3 related announcement at the Worldwide Invitational this year. Between the looks of the runes, the hidden secret files, I'm pretty confident that I may want to dust off my necromancing skills soon enough. Then again, Blizzard may be just playing with the Diablo imagery to fake us out, you never know.
You can peruse the pictures yourself in the gallery below and decide what your position is. Either way, be sure to keep your browser pointed to WoW Insider. We'll have people on the floor of the invitational, and we'll have news on what this splash screen hides the moment it breaks, for sure.
Edit: As of Midnight PST on June 25th, the splash screen has changed again.
Edit: As of Midnight PST on June 26th, we have yet another new splash screen.
Edit: As of Midnight PST on June 27th, we have even more new information and a Lost reference.