Do some Bitcoin wallets fail to support sending to multi-signature addresses?
Sending to P2SH addresses (addresses starting with '3') seems to be directly supported by only Bitcoin-Qt/bitcoind and Electrum at the moment. EWallets using those clients might allow sending to P2SH addresses. It's not directly supported by BitcoinJ, so BitcoinJ-based clients like MultiBit and Bitcoin Wallet for Android probably won't support it until BitcoinJ does. It's also not directly supported by blockchain.info or Armory. Some clients allow you to create raw transactions, which could be used to send to P2SH addresses manually.