It looks like i've hit this issue;
http://code.google.com/p/encfs/issues/detail?id=60

So i've reinstalled boost 1.41 with;
# emerge =boost-1.41.0-r3
# eselect boost set 1
# emerge encfs

but its still broken, and
$ ldd /usr/bin/encfs | grep boost
    libboost_serialization-mt-1_42.so.1.42.0 =>
/usr/lib64/libboost_serialization-mt-1_42.so.1.42.0 (0x00007fb6314de000)
    libboost_serialization-mt-1_41.so.1.41.0 =>
/usr/lib64/libboost_serialization-mt-1_41.so.1.41.0 (0x00007fb6305c5000)
    libboost_filesystem-mt-1_41.so.1.41.0 =>
/usr/lib64/libboost_filesystem-mt-1_41.so.1.41.0 (0x00007fb6303af000)
    libboost_system-mt-1_41.so.1.41.0 =>
/usr/lib64/libboost_system-mt-1_41.so.1.41.0 (0x00007fb6301ab000)

So i'm assuming the problem is its still loading the v1.42 serialisation
library. What's the correct way to that?