Much like Hulu Plus on Xbox requires an Xbox Live Gold subscription on top of the $9.99 monthly fee, some language found on the Hulu Plus site says that only PlayStation Plus subscribers will be able to use the service on PS3.
The HTML source for the Hulu Plus "Devices" page includes specific instructions for the PS3, which are presumably meant to be displayed when the service goes live. The instructions describe how to redeem a code in the PlayStation Store to download the client (which means that Hulu isn't planning to release this as a disc-only service like Netflix -- yay!) with the following caveat: "Note: you must be a subscriber of the Playstation Plus Network."
While this seems pretty clear on the surface, the fact remains that neither Hulu nor Sony have intentionally told the public anything. Sony's Patrick Seybold told Joystiq that "We'll have more info on how and when Hulu Plus will be coming to PlayStation 3 soon." It's quite possible that the specifics of the agreement are still being worked out.