...Doesn't always mean the site is being hacked, does it?

I just got a Google Chrome warning while at GamersGate.

is it due to expire or expired? it usually just means the admin forgot to renew it aslong as it's properly signed atleast

It's a good warning. Somewhere like that won't be allowing that for long, so unless you really want to take some risks. Wait until they've updated.

Email them.

Might just mean the company forgot to renew the certificate. Or install the new one.

Yes, the key there is that as a visitor you don't have any way of knowing. It could even be a case where somebody got their hands on an expired site hoping people would accept it as legit.