According to insider sources the work on Diablo III is only in the creative and visual design stages of it's creation. Due too the supposed complexity of it's structure; It will take more time to make than WoW did. According to some reports Diablo III is suppose to be based on a Virtual World Wide event like WoW is accustomed too. There is suppose to be 3 more characters added, and a complete restructure of the game in it's self. Whether or not these aspects of the game are true or not I cannot get confirmation on, but it wouldn't surprise me with the amout of hiring they've been doing on 3D and creative designing. There rough estimation I have been given from some pretty reliable sources the supposed release date is going to be early to mid 2007. I wouldn't expect it anytime sooner than this. We are basically going to have to wait and see for ourselves, but I would expect March - June 2007 for it's release.
The release information is the only true information I can verfiy from reliable sources, the gaming aspects are rumors from sources who think they have an idea so I take no responsibility for the validity of the information. I am just passing it on to share along the knowledge of the community.