I've seen tons of posts from folks vacationing in October and November who have said they've gotten the ability to customize their bands and make their FP+ reservations. It seems at this point, as is covered in some other threads, pretty much everyone (every resort) is being added to the "test" in those months (i.e. I've yet to see someone who has posted they are going in Oct/Nov and weren't able to). I understand that in a lot of cases no notification was received and the option was simply open on MDE.
As of yesterday folks arriving 12/1 were able to start checking in online. Today, I was able to (12/2 arrival). Curious if anyone out there arriving 12/1 (or later) has been able to participate in MM+ yet? I'm being very impatient about it all. HAH!
EDIT: I read an article on another site this morning that stated testing ends 11/30 and full go-live begins 12/31. I took that to mean that those checking in 12/1-12/30 may not be able to participate. I assume if that's the case it's to switch things over to a permanent production system. Still, I'm hoping the article was wrong or I misunderstood.
NOTE: The article now says 12/1 but I am sure it said 12/31 originally. Still the author of the article states it's an assumption and not anything they learned officially.
EDIT 2: When asking about the possibility for 12/2 arrivals to participate and mentioning the article/information this was the response I received from Disney:
You are correct that testing currently ends on November 30th. However there has been no date confirmed as of yet as to if the test will continue or it will be released to the general public. Any information you may see on third party websites and blogs are not valid. It is possible that either we will extend the testing out through December or even released it by your trip but it is not guaranteed.
We anticipate MyMagic+ will be available to all Walt Disney World Guests over the next several months. Please continue to watch the Disney Parks blog and MyDisneyExperience.com for updates.