The "Single Sign On" & "Fusion Integration" websites on the left are the most built out and useful. These are publicly accessible & hosted under blogging guidelines.
Fusion HCM Integration Community
[By Invitation Only]
The site above is an official "HCM Integration collaboration site" that has details of the REST Services / Atom Feeds in R9 PB6. In June, a sandbox will also be added to allow you to try out the Services/Feeds.
If you are a customer, oracle employee or partner, please email me to request access.
Be sure to also bookmark the following launch off page for Fusion Apps in My Oracle Support - Fusion Apps Product Information Center