Nope, that didn't help.
On all three computers here at work (my linux box using Firefox, my windows laptop running Firefox, and my iPhone running Safari) I hit the "Delete all cookies" link on the web page.
I then proceeded to log in on my linux box, worked fine. Then left the site. I then logged in on my laptop, and left the site. I then logged in on my iPhone and left the site. When I went back to ARC on my linux box, I was logged back out....
Oh well, I guess I need to play with the combinations more to see if I can find a pattern. My ain complaint was more for my iPhone, as it does not have a web site password manager ... the other two machines will auto-fill in my username/password.
Not sure if it matters, but I have the "Remember Me" checked in each of the login pages. And I always load the Forums directly (bypassing the main page).