On a related note, what is the collective wisdom on tipping on corkage fees?

We were at dinner with another couple on Saturday evening and brought our own and had an interesting discussion after dinner about tipping on corkage fees. We had just split the bill, and I tipped our usual (15-18%) on the whole amount. Didn't realise that the other couple had tipped on the pre-corkage amount.

Incidentally, the other couple left their credit card in the restaurant, and the waitress came running out to drop their credit card back, and didn't seem at all upset with their tip.

So what is the norm? Do waitstaff generally expect tips on corkage?