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At a minimum, the sync user needs to have read, write, and replicator rights to the AD subtree for winsync to work. Replicator rights: http://support.microsoft.com/kb/891995 and http://support.microsoft.com/kb/303972.
This popped up on the #freeipa channel (hat tip, Lars Sjöström), and it sounds good. "Add the AD sync user to the AD domain (with adsiedit.msc) and allow the user to do "Replicating directory changes" instead of using the Enterprise Read-only domain controller role."
Also from the channel... "If you're about to perform a winsync the AD-binduser account can be memberof "Account Operator" as well as "Enterprise Read-Only Domain controller" .. no need to be full "Domain Admin"."