UPDATE: rpcbind-0.2.0-3 was just released, and it solves all the rpcbind issues.
UPDATE2: THE BUG WAS SOLVED! Alexander Wirt patched nfs-kernel-server: by removing the linkage of libtirpc, it now uses the previously method of IPv4 binding, thus not triggering the problem. When/if would rpcbind replace portmap - I don't know.
[ This article describes my analysis to a problem found in Debian Unstable(sid) ]
1. The bug (link)
Since the end of December, a change in nfs-kernel-server package caused a change of behavior in some NFSv3 crucial services: rpc.statd, rpc.mountd. NFS is RPC-based, and as thus, it uses an RPC-to-UDP/TCP address translation service, aka port mapper. These services try to connect to port mapper when they need address translation, and since the recent change they first try to do it over IPv6.
portmap, the current widely-used rpc port mapper service, does not support IPv6. This causes these crucial services to die, and NFSv3 fails to start (actually, with default configuration it even prevents starting an NFSv4 server).