From Snowden’s Q&A with The Guardian:
Q: Is encrypting my email any good at defeating the NSA surveillance? Is my data protected by standard encryption?
A: Encryption works. Properly implemented strong crypto systems are one of the few things that you can rely on. Unfortunately, endpoint security is so terrifically weak that NSA can frequently find ways around it.
The /r/onions subreddit had an interesting discussion going on now trying to parse exactly what Snowden means and what the implications are for security and anonymity online.
The consensus is that while encryption such as PGP works, the NSA will have a much easier time simply hacking into a computer once the message has been decrypted.
This matches what is commonly known about the capabilities of most would-be attackers: your encrypted email may be safe but once it’s been opened and read, danger lurks. The weapons available to enter your computer and read the text are diverse and powerful. In case your communications did come under surveillance or attack, protecting clear text is the much more daunting task.