I just got on email from SEOmoz indicating that there will be rate limits for their API:
The API community is growing fast. To serve everyone better, we made some improvements to the way we throttle requests coming into our API. Depending on your application, you may be affected by our new rate limiting system. Here are the technical limits you need to know:
SEOmoz Free API: 1 request for every 10 seconds
SEOmoz Site Intelligence API: 1 request every 2 seconds
If you exceed these limits, you may receive an HTTP 503 response with a Retry-After header indicating the number of seconds you should wait before making your next request.
Will Infinite SEO gracefully handle these HTTP 503's?