We appear to have an implementation of Multisig:

https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/script.js#L329
https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/script.js#L360
https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/transaction.js#L369

However, it is currently not tested, and it's not currently known whether it is in working condition.

Multisig is not really my forte, so I wanted to file this issue and see if anyone was interested in taking a look at multisig and getting it tested. This will go a very long way to getting our Script test coverage to 100% as well.

To sweeten the pot (and to try it out for fun), I'm issuing a 0.03 BTC bounty from the Coinpunk fund for a pull request to improve this!