I’m not going to try to write a how-to on this, but suffice to say that it finally works!
I configured Samba3, PAM, and Kerberos on Ubuntu 9.10, and I can share folders to Windows client workstations using their existing authentication to the domain controller. I can even set ACLs that seem to be obeyed, and use Windows group membership to determine access rights.
Here are some links with relevant information:

The Samba WIKI (geared towards RHEL, but still helpful.)
How To Integrate Samba Using Active Directory For Authentication (geared towards Gentoo linux, but still helpful)
Ubuntu Samba info

I’ll certainly try to answer any questions you’ve got, just post ’em.