Rasara server is based on Samba 4 alpha. If you feel good about being
early adopter you can get the latest Samba4 alpha. Actually Samba4 is
pretty stable and supports native NTDS replication along with all
Windows Support Tools. You have "samba-tool vampire" (equivalent to
dcpromo) which you use to initially suck the data from the current DC.
Nevertheless Samba4 supports Win2008R2 Read Only Domain Controller
role so you can use it as additional DC that could be a read point.
http://samba.org
#samba-technical (IRC on freenode)
Good luck!
This is the latest alpha announcement:
http://permalink.gmane.org/gmane.network.samba.internals/54714