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.
More information about guilds will become available when Starcraft II nears its launch date, since Blizzard has created numerous new features for Battle.net 2.0 that they are not yet ready to reveal. Expected features include improved friends lists and easier online communication, but how these and other features will be integrated into user Battle.net accounts, and how those will work with possible Diablo III guild features is not yet known.