As an airline employee:

The ID when checking in is a matter of corporate policy - we at B6 are only supposed to check when the passenger is checking a bag. But 10% of the time when I'm helping in the operation I forget to check. It's no big deal. If someone forgets to check an ID (especially when the customer is having some friendly banter with us) it's nothing big. And when IDs are checked, you can pretty much show anything (Sometimes I just glance at the picture, let alone look at any information). The important ID check is with TSA.