Encrypt your data with TrueCrypt
TrueCrypt is an open-source disk encryption software for Windows, Linux, and Mac OS X.
Ask yourself the question: “If my laptop gets stolen, is there anything on it that I don’t want to be found?” If the answer is yes, use TrueCrypt. Create with it a virtual encrypted disk or encrypt an entire partition.
Troubleshooting (update 20110301)
Recently, I noticed that TrueCrypt cannot unmount some volumes. The error message says: “device-mapper: remove ioctl failed: Device or resource busy”.
Solution: in the TrueCrypt GUI, go to Setting -> Preferences -> System Integration -> Kernel Services and check the box “Do not use kernel cryptographic services”.
I found this help in this thread.