Oded implemented this earlier today. Right now, the default rate-limits apply to new users on every site (where "new user" is defined in a way that requires some nominal participation on that site), forcing a waiting period between posting a question or answer after having previously posted one anywhere else on the network.
These are not particularly onerous rate limits, but should suffice to let the spam-handling mechanisms kick in before excessive quantities of messages can be generated.
At present, this overrides site-specific rate-limits, which is less than ideal. Hopefully Oded will take a crack at correcting this, and also chime in here when he does for a chance at that sweet bounty goodness.