"So what happens if I set up a domain controller on eg Server 2K8, add in a bunch of other domain controllers using Samba 4, then remove the original Server 2K8 machine? Does it still work?"
AFAIK, it would continue working, in just the same way as if you had added a load of 2K8 DCs then removed the original. Someone else can probably confirm this.
"More importantly, if you're only using AD for authentication - what happens when it comes to CALs if you're using an AD running exclusively on Samba4 installs on non-Windows boxes? I suspect Microsoft's stance will be that you still need CALs on either a per-user or per-machine basis, but it's an interesting question to ask..."
I'm not sure which way round you are talking here.
If you mean a Windows server with Samba clients, I believe you still need CALs.
If you mean a Samba server with Windows clients, you don't.