Keep an eye on your email and payment options you have tied to Steam. Details on why this is happening aren't clear yet, but logging in to Steam will show you someone elses account! Your email, paypal email, Steam wallet balance, order history, last 4 digits of your phone number, billing name and address, and possibly other information may be exposed. I just checked the steam account page and I got someone elses account, clicking the account history and license history are showing me the information from 2 OTHER accounts.
This is super fucked and you can't really do anything about it because you can't access your own account details to change them. Valve is going to have a lot of explaining to do.
EDIT: It appears Steam has just gone offline.