The D3 Team has said that they hope to provide guild support in Diablo III. They can't say much about how that might be provided because they don't yet know. They have not spent much time working on it or other multiplayer, online-only features yet, with so much work yet to be completed on the basic game.
During a press conference [1] at Blizzcon 2008, Jay Wilson commented on guilds. He confirmed that guilds are on the drawing board, and that cooperative online play is a cornerstone of the series. He also noted that guilds will be supported, but not mandated. They are not planning to include "raid" style content in Diablo III; monsters or dungeons that could only be enjoyed by large groups.
The D3 team has strongly hinted that they've got more to say about Guilds, but can't talk about it yet, since the features are tied to the improvements coming in Battle.net 2.0. These are expected to include improved friends lists and easier online communication. More information will become available when Starcraft II nears its launch date, since that game is going to be released before Diablo III, and Blizzard is going to debut a lot of new Battle.net features with the title.