The only ones that are being officially let in before the Grand Opening in June are the Harry Potter vacation package holders and onsite guests who booked room only BEFORE March 25 2010.
This isn't to say that others won't be let in at some point but if you call Universal they will make it clear that you won't be allowed in without one of those two things.
When you check in you will be given what you need to get into the area. So Universal must be " watching" that area for non-invited guests.
I have onsite room for May 30-JUne 6 ( HRH ) and believe me I freaked when the date for June was announced. After no less than 6 calls to Universal and three e-mails I am confident that I will be getting in. One rep even said I was lucky to be able to preview it then, less crowed and more intimate. The plan is to have everything open but of course techie stuff can happen.
Again, this isn't to say that " day" guests are going to be turned away but since no one knows I would hate for someone to plan a vaca there only to find out you can't get in. Even booking now onsite doesn't guarantee entrance, only pre-March 25th bookings.
I got soooooo lucky. Hard Rock Hotel at 150.00 a night PLUS Harry Potter. I am usually not lucky at all so this is really great!!