I've been using FF3 and hearing about these warnings for a while now.
Just yesterday, I was resurrecting a seldom-used internal server at work and realized that its self-signed SSL certificate had expired, and I'd have first-hand experience with the new, dire warnings.
Frankly, I don't understand what all the fuss is about.
It's a few more clicks, but not that many. And you can make the exceptions permanent, just as before. So it's probably fifteen seconds of work once every year for every self-signed website you visit. I think we can manage.