Apple:
There is no limit to the number of photos you can upload to My Photo Stream over time, but iCloud limits the number of photos that can be uploaded within a given hour, day, or month to prevent unintended or excessive use.
Update: This might not be so new after all.
I think this article highlights how confusing Photo Stream is, and why it’s probably best to view the service as a temporary conduit to move photos between devices, not as a long-term storage solution.