I was logging into the heroku website.
To test the CLI I ran `heroku logout`, then `heroku login`. It brought up a browser window with "Application error.", so looks like it's indeed down.
EDIT 1: Funny timing because fly.io is also down (for me, anyway) [1]
[1] https://news.ycombinator.com/item?id=36243345
EDIT 2: I tried heroku login again, and this time get "Invalid Request".