This is so hyped up. The actual birthday is 7/17, and I think a lot of people are afraid that they won't get in unless they're staying there, or they just want to immerse themselves in the magic, so those hotels have been booked up for awhile. The rooms become available as deposit due dates pass and $ is not paid. I also imagine that around 30 days before some rooms will become available, though not many.
Personally, I do not anticipate a hard ticket event. There are a lot of people coming in from out-of-town, and it doesn't make sense that Disneyland wouldn't let them know up front that they'd have to pay additional to get in the park, or that 17 July admission would have been a part of these packages some time ago.