This has come up before. I think the consensus was that:

In an emergency, the ER isn't going to mess with anything like this. The will look for a medical bracelet, but that is about it. Also in an emergency, they don't need a lot of details, just the high level stuff like your Dr.'s contact info, meds you are on, drug allergies, and serious existing conditions. This information needs to be easily accessible -- not encrypted, no need for special programs, etc. Probably also means no computer/pda. Basically, what you need is a medical bracelet. :-)

If it isn't an emergency, they probably still aren't going to mess with it. It would be up to you to provide the information. If this type of device is handy for you in that context, great.

Note there is a good free encryption program called Truecrypt which you could use to encrypt any USB type drive, and it does have a "portable" mode which you can run the program from the USB drive itself.

Personally, I think it is helpful to have various personal data in a portable mechanism. I also think it is important to encrypt said data. This also goes for sensitive data on your home personal computer or laptop. Encryption is your friend! (Backups are your friend too!)

I do have a number of USB drives which I keep encrypted, but for data I really want to have with me I save it in an encrypted program on my iPod Touch (eWallet, which allows you to sync to your PC) since I have that with me and it is self contained (no external PC needed).

Note, when encrypting data on a thumb drive, consider partitioning the drive into an encrypted partition, and a non-encrypted partition and keep a copy of the encryption software on the non-encrypted partition. And remember, bigger is better. This stuff is cheap, why skimp? You should be able to pick up a 4GB stick for ~$20 at your local office store or electronics store. Considering the possibility you might break it, get two!

Encrypted USB drives allow you to have large amounts of easily transportable encrypted data (think backups of all your important data from your PC). But something like encrypted files on a PDA or similar might be a bit easier to have important data at your fingertips.

YMMV.

-john