As was mentioned, truecrypt will solve this problem.

If by some reason the morons at CBP find a judge who will compel you to reveal the passcode (5th amendment considerations aside), you use a "hidden volume."

If that does not work, you use a hidden volume with a keyfile that you have on a USB stick. You e-mail this to yourself and temporarily delete it off the USB stick while passing thru customs. You can then claim, honestly, that you do not have the keyfile on your person or available to you at that time. I'd also recommend a service such as hushmail for the e-mail portion, because it'll be encrypted again (and is outside the US). Or, literally mail yourself the USB key with the keyfile.

This whole deal with customs is not an insurmountable problem by any means if you are willing to spend a bit of time on data security.

Now, that said, if you are doing this to sneak child porn around, you (IMHO) deserve to get caught.