Have you confirmed that gmail hasn't lost your POP settings? On gmail's settings page, under "Forwarding and POP" you can enable POP and set some other stuff. It's easy to not click the Save Changes button - of course, I'd never do that myself <cough, cough>
You could also use wireshark (formerly ethereal) to sniff the traffic. Because it's encrypted you won't see the data, but you will see if pop.gmail.com responds to your connection attempts.