Jason,
I believe that your answer is accurate but only with full Office365 instances. I say this because when I try to add a CRM Online trial to my Office365 trial I get this error:
I wish the error was more specific, I don't want to advice a customer to purchase a full Office365 account (thinking the error is due to the third reason on the list) and then get the same error based on the second reason or something like that.