I'm relatively tech-savvy, but I'm no security expert. To my understanding, an invalid SSL certificate is only a problem if you're going to provide some sort of potentially exploitable information to a website and you are not sure that the website you're at is truly owned by the organization you believe it to be.
I ask because my workplace uses content filtering that makes every SSL cert invalid. The browser sees the website as originating from the content filtering server on the network rather than the actual server the website is being served from. I'm tempted to simply turn off certificate checking altogether in my browser (Firefox) because it's not doing anything for me other than creating hassle, but I wanted to check to see if there's some facet of the issue I might be missing? I'm smart enough to ensure that the website I'm visiting is the website I think I'm visiting without the confirmation of the cert, so based on my understanding, I shouldn't have any problems.
Best Answer
Basically with that kind of proxy, your employer can see even banking information and such via SSL because they have an unencrypted copy via the proxy. Your computer is requesting a webpage from the proxy server, and then your employer's proxy server is requesting the pages from the destination on your behalf, and the proxy software gets an unencrypted copy because it is in the middle. So the proxy can see the contents of every web page you see. The only way SSL is secure is when your PC and the destination PC talk directly via SSL.
Your browser is correctly warning you that your information is not secure. I think that the connection between the three points is still using encryption, so the whole world can't see it - just your employer.
One note to remember, even with SSL turned on properly, your employer can still see the URLs (in the browser address bar) that you go to. Most search engines like google place a lot of information in the URL (words you searched for, etc).