Well, at 6:30am (EST) this morning (roughly) Blizzard unveiled that they are working on Diablo III for the PC/Mac. With the unveling they showcased the game itself - as two characters (the Barb and the new Witch Doctor which seems a lot like the Necromancer from DII). Let the discussion begin!
----------------------------Rest is previous speculation
I think this is pretty much set in stone, but this Saturday I would expect to see Diablo III revealed by Blizzard :)
The splash image on http://blizzard.com is enough evidence for me to start jumping up and down already :Jump:. The image is of some crystal or ice with something large behind it - that large something having a lot of red hue to it. Upon closer inspection you can see a rune image/symbol from Diablo II in the upper right of the image (not hidden at all - Hel Rune image, one of the lower runes from D2:LOD). The other rune/symbol is not familiar to myself. There's more proof I'm sure (just go to any of those Diablo or WoW fansites, they seem to always obsese over these things).
Each day the image progresses more towards revealing what is behind it, so check tomorrow to see an even clearer picture :)