Their status page just says "Trello is slow", which implies it's still working in principle. However, I haven't even been able to authenticate for the past 45 minutes or so - for me, it's broken, not slow.
I do see that they've had some auth systems maintenance on Sunday... could be related.
Github down I can cope with, but having started leaning on this more for day-to-day task management, downtime really sucks.