I understand that banks are lowering credit limits on unsecured cards and perhaps closing unused cards (although I've heard of this happening far before the current credit crunch), but I've got loads of anecdotal evidence of people who pay their (unsecured) cards in full monthly not having their cards cancelled.

The bank wants you to pay, after all. The interest calculation is intended to make you pay quicker, and the bank knows the more interest you accrue the more likely your debt to them will become bad.