* Any way to tell someone has sent you a multi-sig transaction
"Patches welcome."
A watch-only wallet that has a bunch of public keys (and multisig groups of public keys) is a good idea. But first you'd need multi-wallet support.
There is a pull request that adds watch-only addresses, but I think that is the wrong way to go. Mixing up fund that can be spent with funds that cannot be spent (e.g. because they're a multisig escrow you want to watch) is a bad idea, and as soon as we have hierarchical deterministic wallets we'll want watch-only wallets that are derived from a master key (or a set of master keys in the case of multisig) where the local bitcoind doesn't have the private seed for the key.