Gabe asked me to repost this here - it's an article I wrote summing up what we know so far of the MyMagic+/Fastpass+ system, along with some speculation and relative confidence of such:
Some details of the forthcoming FastPass+ and MyMagic+ programs at WDW
A quick rundown - most of the information is taken from the Terms & Conditions page on the My Disney Experience web site and app (seems the iOS app has been updated for FP+ features although they don't work yet - Android app has yet to be updated), along with statements from Tom Staggs and his minions, and the EVP of NextGen, Nick Frankin. Everything is subject to change though (like how they already changed the RFID PIN requirement after tests raised serious issues).
MagicBands will take the place of KTTW cards for resort guests (although you _may_ be able to opt for a more ticket-like RFID card), and will handle everything that KTTW cards do, plus PhotoPass (or PhotoPass+, some sort of new package), and also help provide additional "experiences".
FastPass+ will allow guests to choose attractions return times and other experiences such as exclusive fireworks/parade locations up to 60 days in advance. Current limitations are a maximum of three pre-booked experiences (and possibly in total...) per day, and only for a single park per day. Disney expects to be able to offer additional things, such as additional FP+, depending on availability, etc. It appears it will also limit you to a single FP+ per attraction. If you want to ride more than once, you need to use the standby line.
Attractions are likely to be tiered, with "E-tickets" being in one tier with possibly only a single selection allowed.
Guests using FastPass+ will NOT be able to use regular Fastpass, which will exist in tandem at least for a while as eventually every guest will be able to utilize FP+ (all tickets will be RFID). It is unclear if there will be any FP+ available during the day like with FP, or will it only be able to be booked in advance.
Up until the first FP+ experience time, you can change all of the experiences and park. Once you hit your first experience time (or it expires, it is unclear), you can only change the remaining ones and it appears only for the same park.
All guests will eventually be able to participate in FP+ (and very likely it will be the only option once all guests have access). Guests will have some way to tie a pre-purchased ticket to their account in My Disney Experience to be able to pre-book FP+. It will NOT be restricted to resort guests (no is it expected at this time that resort guests get any extras), although it will likely be rolled out in phases, with one or two resorts' guests getting it first, etc. I hear Boardwalk Inn is the first to be converted.
Still a lot of questions as to what benefits AP holders will get. Initial rumors had them getting a fixed number of FP+s available per quarter. Hearing some rumbles that they are re-thinking it.
Guests will be able to use the My Disney Experience app to access their account and book/change their FP+ experiences, even while in the park (once park-wide Wi-Fi is completed). Guests without smartphones will be able to access their info via kiosks throughout the parks and resorts. It is expected that the app will also allow Disney to send reminders and special offers to guests based on where they are, etc.