I'm not certain what time they let folks in but I can pass along my experience from December 2002.
Our PS was CRT for 8:50 am. When I booked it I was told to be at the Magic Kingdom by 8:00am. That seemed a little too early but she had said it was just to make sure I got through the lines of people waiting for opening okay.
So, on our day, also not an EE day we arrived to the TTC but before we could get to the monorail line we were stopped by a CM who made us show our tickets and PS number and time. She didn't have anything to check it against but I guess she just wanted to be sure we had one. I get the feeling she also thought it was too early to be heading that way because she checked her watch, frowned and then said "Oh, all right, go ahead". The ferry was broken so everybody was in line for the monorail.
Finally got to MK, went through security and approached the lines on the right side. I was stopped by a CM who asked if I had a PS and for where. Apparently, she misunderstood me because she said I had to wait for park opening and walked away. I went back over to her and said I didn't think she understood because my PS was for 8:50, before the pak opened. I then showed her the paper I had written the number and time on. Then she said "Oh, I guess you do have a PS" and let me get in line.
Moral of the story -- go EARLY, have your tickets in hand and your PS time and number written down to show.
I'm hoping it was just a fluke that we had a hard time getting in. I've never heard anybody else have a hard time with an early PS so it's probably just our bad luck.
Enjoy the CP, we had a PS there too and it was wonderful. Good food, good character interaction and it's beautiful inside.