After much futzing, and multiple clean reinstalls of the beta, I think I have found the problem: when I installed Remember the Milk, I did not select a list, left it blank to include all lists. Once I selected my main list (inbox) the behavior stopped. At least, as of now. WHOOO HOOO.
So, for future RTM users, you must select at least one list in order for tasks to sync properly.
Rainy, just for my own interest, I'd love to know if you can repeat this with your setup-- ie, remove everything from the "lists" category in RTM calendar setup, and see if you get the re-appearing tasks.
I'm off to register.