There are a lot of unwritten rules when it comes to advertising. Getting a supermodel to talk about why people should use Weight Watchers -- not a good idea. Asking Bill Gates to hawk iPads -- not a good idea. Lenders incorporating humor into their marketing campaigns… that's not such a black-and-white debate. There are no unwritten rules about that. Should there be?

Given the furor over the role that financial institutions played in the economic meltdown, about the only way a lender could be funny and still pull it off would be if the humor were self-deprecating. Having taken billions of dollars from taxpayers and then sending a message that everything is so great that it's OK to have a good time may not be the best way to win new business.

Some videos recently posted to YouTube by Sovereign Bank -- owned by Banco Santander, parent of one of the nation's largest auto finance operations -- attempt to walk a humorous line using another tried-and-tested humorous delivery vehicle: babies.

The video has nothing to do with Sovereign Bank. There is no dialogue, no voiceover, no Sovereign logo. It's just 57 seconds of a baby appearing as though she's grocery shopping. The only mention of Sovereign is in a description of the video, which says, "Shared with you in mind, from your friends at Sovereign | Santander." The video has become very popular -- it has 130,000 views since it was posted last week -- which will no doubt help promote Sovereign.

The video and ensuing discussion raises the question as to whether lenders should ever be funny, especially when dealing with such a serious subject. Homes and cars are likely to be the two largest purchases that people will ever make, and when a bank tries to use humor in promoting itself, the message can come across as undignified.

There's another commercial making the rounds, this one from Volkswagen. In it, a man in a car dealership is on the phone with, presumably, his wife. He's telling her about how he negotiated a great deal on their new car, 0% financing for 60 months. The salesman was only offering the rate for 36 months, the man tells his wife, but he drew a line in the sand, and got the deal for 60 months. The camera cuts to the salesman, standing in front of a sign that says "0% financing for 60 months." The gist? That the deal is available to everyone. This is a good example of self-deprecating humor that sends a clear message: The customer won.

The decision to use humor in a banking commercial or advertisement is anything but a slam-dunk decision in today's highly polarized, everything-ends-up-online culture. It should not be taken lightly, but should be used lightly. Let's make that a new unwritten rule.