Sometimes, I randomly get the above error. I'm a KS backer who converted my key to Steam, so I'm not sure why on earth I need to log in *again* when Steam is already running, but definitely not sure why it would fail.
Cola is right, Steam should automatically log you in. Every now and then, if the server is busy at the time Steam is requesting your login, it might time out. (Happens when servers are really busy like PA's have been) If it keeps occurring over the next few weeks, give this thread a bump and we'll convince them something is amiss.
You are loging with the game into the Ubernet Servers. Your Ubernet Account and Steam account are seperate.
Separate but linked. Steam automatically logs in UberNet using your linked accounts. (That's how it worked when I still used Steam, anyhow.)
Every time I start the game, I have to connect to paly fab. Everytime I start a match, the first message is "login accepted". The login happens automatic in the background (no entry of information), but it is there. As I understand it the Ubernet servers themself need that login before it let's you/your client create the game. You have to verify that you are allowed to create games onteh Ubernet servers.
Yes, that is your Steam account automatically logging you into the game since the accounts are linked. It's a hair longer, but just as easy so long as it works. (Even the UberLauncher can be funny sometimes. I've logged in once to play the game, then again to access the main menu. Silly networks.)