That means that during the Hulu Plus preview period, you need to be a member of PlayStation Plus to access the service on your PS3; however, with Hulu Plus currently in its "preview period" and "launching to the public very soon" and the PS3 app nowhere to be found for PlayStation Plus subscribers, it's not clear how many potential users there will be on the PS3 before it goes public. Full statement from Hulu after the break.
With the launch of Hulu Plus there have been many rumors started and we are trying our best to remedy the situation. Specifically to your question, the Playstation Plus service is only a requirement during the preview period.
Once the preview period ends you should be able to download the Hulu Plus app but that will still require the Hulu Plus subscription.
Right now we are limiting the number of users but Hulu Plus will be launching to the public very soon so stay tuned!