I was having problems to, but was able to get around it. So if you're have problems changing your Facebook profile picture try this:
1. Go to your "Profile Pictures" album.
2. In the top right corner click on the "Change Profile Picture" button.
3. Then add an image file from your computer hard drive. (NOTE: It would not let me change it to a picture already existing in a FB album.)
This was the only way I was able to change my profile picture on Facebook today, so hopefully Facebook get things fixed so you can make existing photos your profile picture.