It's been in Beta since the summer. I think they just made it publicly available yesterday.

There is an implementation guide available here:

http://developer.authorize.net/guides/

(towards the bottom..."Customer Information Manager (CIM) – API Guide")

It looks like it requires a couple XML calls to setup a customer profile, then a very simple XML call whenever you want to charge them. Looks great to me.

Unfortunately no PHP source code examples yet, as far as I can tell...