Do some Bitcoin wallets fail to support sending to multi-signature addresses? The Satoshi client has support for multi-signature transactions through the JSON-RPC API since version 0.6.1rc2.

My understanding is that a multi-signature transaction creates an address which is special only in that spending from it requires multiple signatures, and its prefix is '3' instead of '1'.

As Gavin said on bitcointalk.org, "you can then send funds into that 2-of-3 transaction using the normal sendtoaddress/sendmany RPC commands, or the GUI (or anything that's been updated to recognize multisig addresses)."

What I read from that is that all wallet clients and web wallets might not yet support sending to addresses with a '3' prefix. How wide is that support currently?
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.