I accept that things that I share on the cloud/with big tech companies are probably accessible by someone. I don't have any guarantee that the top people at Google/Yahoo aren't looking through my stuff--they certainly have the ability to do so.
As long as you're storing your data in a service like this, someone will have access, probably multiple people. This new PRISM stuff just tells me that a few more people have access, it doesn't really change much for me.
If you are concerned about this type of privacy I'd suggest living like Stallman then. It's not as if you don't have methods to keeping your data private.