If you are a broadcaster, you should check your email address that was registered on JTV. If you see password reminder emails, it is likely that an unauthorized user accessed your account. As I do not know if JTV are able to roll back any unauthorized changes, I recommend that after JTV comes back up, you thoroughly check your account settings for any unauthorized changes to your data - most notably your email address, chat channel moderators / ban lists and authorized sub-streamers. I would recommend you do this even if you did not receive any password reminder emails.
I will update this OP with any official update from JTV as needed.
UPDATE (15:45 EDT): Justin.TV is back online.
15:53: And down again... and up. Seems they are still working on things. System is still not entirely secure, don't verify your account information just yet.
UPDATE (16:15 EDT): JTV confirms no passwords have been compromised, only temporary account access was possible.
UPDATE (17:45 EDT): Justin.TV is back online (heavy load), all accounts have been logged out so the system should be secure now. I'd recommend broadcasters verify their account settings are all normal now. And... down again.