Doesn't the Owner's Manual say to use the black plastic key every so often to keep the battery charged and the chip working? I'm pretty sure that's where I read it (either X5's or the 3 Series) because I used to do it every 6 months or so on both cars.

No I think those are just the regular keys - and thats to charge the battery that is used when you remotely unlock/lock the car. Since the plastic key you have to turn in the door there should be no need for a battery.

The plastic one I believe just has a chip, which is most likely rom - which means electricity or not it keeps its data. Plus the plastic key has no metal contacts, so how would it charge?

Interesting post, Chip.
with Wagner- Russian Roulette just to keep the key in your pocket?
CNet just did a good article the other day with some more info for those interested.
Intro:
Let's say you just bought a Mercedes S550, a state-of-the-art, high-tech vehicle with an antitheft keyless ignition system. After pulling into a Starbucks to celebrate with a grande latte and a scone while checking your messages on a BlackBerry, a man in a T-shirt and jeans with a laptop sits next to you and starts up a friendly conversation: "Is that the S550? How do you like it so far?" Eager to share, you converse for a few minutes, then the man thanks you and is gone. A moment later you look up to discover your new Mercedes is gone as well. Now, decrypting one 40-bit code sequence can not only disengage the security system and unlock the doors, it can also start the car--making the hack tempting for thieves. The owner of the code is now the true owner of the car. And while high-end, high-tech auto thefts like this are more common in Europe today, they will soon start happening in America. The sad thing is that manufacturers of keyless devices don't seem to care....
Full Article Here