Account creation[1] was enabled on Wikimedia wikis a few weeks ago.
There is a bug[2] in the implementation that returns status codes in
all lower case ('success', 'needtoken') rather than in CamelCase
('Success', 'NeedToken'), which is what all other parts of the API
use. There is a patch[3] that will make action=createaccount behave
similar to the other actions.
If you have code that uses action=createaccount, you might have to
tweak your client to uppercase a few characters.