They potentially can; commercial firewalls can man-in-the-middle HTTPS traffic with a locally signed and organization-computer-trusted SSL certificate.
How does that work? I thought all verification of certificates was done in the browser...
IT installs the corporate MITM certificate on all of their computers so the browsers accept them as valid.
Would this still affect the iPhone 4S though? If I understand this all correctly, I think that corporate IT would have to install the self-signed root cert on your phone for Siri to be MITM'd. There's no reason for your phone to trust it otherwise.