Users of MyBB should not be concerned about their own installations. There is nothing to indicate the MyBB software itself contributed to the hacking in any way.
We hope to restore access to all services in the next 12 - 24 hours. At this stage we don't believe our database was compromised, however we recommend users stay vigilant to unauthorized access of their accounts.
Updates:
11:15 AM GMT: We are pleased to say that we have control of the MyBB.com domain again. MyBB.com will be accessible again shortly, but services won't be available until we have thoroughly assessed what, if any, access the hackers had to our server.