It is possible, but it requires some setup. Here is how it is done, and how you can tell.
On a corporate computer, where software updates are pushed from a central location, it is possible to send to your computer a "trusted" certificate that will be stored next to the trusted certificate of say, Verising or Entrust.
Your company's proxy will hold the private key of that certificate.
When you visite a HTTPS web site, like https://mybank.com/, the proxy will put itself in the middle. It will establish a HTTPS connection with your browser generating on the fly a certificate for mybank.com. It will replay (and possibly monitor or log) all you traffic on a new connexion, from the proxy to mybank.com.
You can tell if this is the case by looking at the padlock icon. If you see that the certificate for mybank.com was issued by acmesprockets.com (your company), then you know they can see your "encrypted" traffic. You can also check with a computer on another network (at home) and compare the results.
I know that BlueCoat supports that configuration, and I'm sure others do, too.