I am using Opera here at home, yes, but I've also seen it while at work, where I am forced to use Firefox.
The last couple days I've been getting randomly and completely logged out, as well. I'll log in, read posts for maybe 5 minutes, then type a reply, hit Submit, and get the "You are not logged in!" vBulletin error page. Hitting back, and reloading the page shows that I am indeed logged out. Clicking "Login" does actually require me to log in again.
That'd indicate that either your network session actually got logged out from another site on the network and that your wowace one was stale or that your cookies got wiped.