List of minimum operating system requirement for Geneva server?
This isn't finalized yet.
I think server 2003 will be supported as well as 2008. However, I don't know which edition will be required. ADFS v. 1 required enterprise edition. I'm not sure if that will be the case for ADFS v. 2. You'll also need a SQL Server database. I don't know if express will work w/ the final version or if standard or something better will be required. You'll also need certain components of IIS installed (e.g., the authorization module, ASP.NET, etc.), .NET 3.5 SP 1, and WIF. See Geneva Server's docs also.
Again, none of this is finalized to my knowledge.