FTB was/is down earlier. Since I couldn't log in, I grabbed the tekkit pack to try out the new Fekkit server. That connection failed (end of stream) and since then when I try to login to vanilla I get the "you are logged on elsewhere" message. I've been googling but can't find anything legit that says how to solve this. Any ideas?

The FTB server is frozen / crashed and you are logged in to the FTB while it happens. In the news page you can see that our names are still in the FTB online stats. You can't play / log on other server until FTB fixes.