I saw this commercial this morning for one of those credit cards (it was Discover, I think) you can attach to your keychain. They're kind of amoeba-shaped, and they come with a cover for protection. I'm sure you've probably seen them before - they're not that new.
I don't really understand the point of this product - I mean, is a credit card really that burdensome to carry? And doesn't the cover add enough weight to make up for the fact that the card is smaller? They try to make it seem like it will save you time, but what - maybe ten seconds if you've already got your keys in your hands? If you have to dig them out of your bag, it doesn't seem like it would matter at all. Plus who wants an extra thing on their keychain? I like to keep my keys as bulk-free as possible. I suppose if you have a card on your keys, it could possibly save you from having to carry your wallet at all, but the commercial featured a man, and if you assume they're intentionally trying to market to men, then don't men always carry their wallets anyway?
Anyway, the preceding paragraph is not the real point of this post. The fact that I don't see the advantage of the card doesn't mean other people might not really like it.
My real point is that I think this card is actually very dangerous. Credit cards have your name on them. Think about it - if you lose your keys, and someone finds them, or if someone steals them outright, they could look your name up in the phone book and easily find out where you live. They've now got your keys and your address. You might as well leave your front door open and invite them in to rob you.