Diablo from Diablo I looks a lot different than Diablo from Diablo II. Perhaps they may alter him again (wings, armor, etc.).
But one thing I heard in BlizzCast 2 that cheered me up is that Samwise Didier said that at Blizzard, they don't like to change the look of a villain or hero, and he said something like, "No one would change the appearance of Darth Vader, and we have the same feeling about that."
Those aren't his exact words, but I remember it was something along those lines. I think he was referring to Kerrigan
For those who don't play World of Warcraft, the demon you see at the Blizzard main page is Kil'jaeden - former lieutenant of Sargeras and Lord of the Burning Legion. The main topic of the Patch 2.4 (Fury of the Sunwell) is the summoning of Kil'jaeden into Azeroth by using the Sunwell as a portal. Kil'jaeden is the final boss of the 25-players raid dungeon.
Just thought I point out that not only is June 29th the anniversary of Diablo 2. It is also the anniversary for Diablo 2: LOD. Diablo 2 came out june 29th, 2000. Diablo 2: LOD came out June 29th, 2001.