Is there a rate limit for making requests against the DataCite APIs?

The current status is that we do not impose per account rate limits. However, there is a top level hard limit imposed by DataCite's firewall which is based on IP address, and this is around 3000 requests in a 5 minute window. There is also an upper limit for requests that come via Content Negotiation of 1000 requests in a 5 minute window.

More information about the DataCite API and Content Negotiation