Instead of thinking about encrypting your full disk, I would recommend you instead look at specific data in a secure way. Meaning—for example—I am using Mac OS X. My main hard drive is not encrypted. I use GPG/PGP for some mail interactions, but it is mostly in the clear. What I do care about and secure I keep stored on encrypted disk images I can create within Mac OS X’s built-in “Disk Utility.”
If I need to decrypt that data, I just mount the disk, enter my password and access it. And when I am done, I just unmount it and there that is.
Now let’s say someone stole my system. Maybe they could glean some info from my mail, but not anything worth anything… They would not know my keychain password. And as for encrypted disks go, even less so… They wouldn’t be able to mount those disks and do anything.
So my advice to you is even if you are not using Mac OS X, maybe you should reassess your overall security mindset and footing. Remember: Security is a mindset more than it is simply one “mega” tool you install or setup. Encrypting your whole boot disk—which can be at least 50GB for most people—is a bit overkill. But carefully maintaining and securing the data you feel is most sensitive is a more practical way to go.