samba - Mar 2010 - Questions about nsswitch/winbindd_dual.c behavior

I'm using samba-3.0.34, and I have two questions:

(1)
I'm not exactly sure how the asyncronous behavior of winbindd_dual works.
Does it fork a child for each domain? each domain controller?
Or does it fork a child ad hoc whenever it needs to handle a request?

(2)
I'm joining to an active directory domain using "net rpc join"
fine,
but wireshark
shows winbindd doing a search to find all domain controllers.
Since I explicitly gave a primary domain controller in smb.conf and when I
joined, why is it looking up the other domain controllers?

Thanks,
- Jeremiah