Here's the simple truth of the matter...what midnitrcr says is correct to a point. But it's quite simple and I'll blanket it with a general/possibly naive statement.
'If you told me that I must transmit information from point A to point B using digital technology, then I have confidence that I can send a message that the government will not be able to crack.'
No doubt there's lot's of shitty tools out there which range from easy to difficult in getting around, but there's also plenty that you're not going to crack. My concern would not be in sending the message...in fact that's where the biggest security is. As was said above somewhere. You attack the person, the computer, and so on.
Since I love giving examples, here's a perfect one. Why can you and I burn DVD's? They are, after all...encrypted. The problem is that the encryption was broken (by a guy known as DVD John) because the end points were insecure. Somewhere you have to decrypt that message, and that's the sweet spot where you strike. Computers are better than DVD players, and it was really partial human error that the code was broken for DVD's.
If I hand you an encrypted file, I have no worries...it's only in the endpoints that I worry. If you are even a semi-organized terrorist organization...you will have the sense (I hope) to learn how to do it right to avoid being detected. But we also don't have to bust a terrorist simply by unencrypting the message. If I am the government, and I suspect you...I would simply watch where the traffic is coming from and then pull your location using something like your IP. Then it's much faster and easier to go steal your machine or interrogate you, or whatever else (just thinking out loud on this one). Then again it would be an interesting test to see how PGP Whole Disk Encryption stands up to the test