Do you need to quote the second item if you are running this from the shell? Not by the computer to try it.
e.g. a ' before the {
?
like:
bitcoind sendmany "" '{"mvTt8hav6e9ESjSrXJ1yaJhyULHv8ywcN7":50}' 1 "To the Faucet"
bitcoind sendmany "SomeAccount" '{"myeTWjh876opYp6R5VRj8rzkLFPE4dP3Uw":10,"mikZVLuasDcY1Jmph3rqgT1NXfjB1srSEc":15,"mvTt8hav6e9ESjSrXJ1yaJhyULHv8ywcN7":50}'
See:
https://bitcointalk.org/index.php?topic=4254.0