Are multi-sig M-of-N transactions with N > 3 supported?
Standard multisig outputs have max 3 keys, but it is possible to add up to 16 keys.

This limit is probably a result of the hard limit of output script size.

EDIT: source: I don't remember where I found this information. (I implemented it in Dartcoin and put that in the documentation.) As an alternative source, check out Mike Hearn's BitcoinJ implementation here.