Samba 4, currently the alpha development of Active Directory for Linux and Unix, has been in parallel development with Samba 3. Allison says "The ultimate goal for Samba 4 will be to merge with the Samba 3 file server code, something that is called "Frankie" internally, after Frankenstein, the merging of the body parts...

The client side of Active Directory is relatively simple, and we've had that for years. The server side is a nightmare, because it involves so many different things - it's Kerberos, its a DCL PC server, its a DNS server, it's a file server... It's a massive conglomeration of things that have been glued together... and it's all standard protocols, except for the bits they've made non-standard..."