The bigger problem is being infected by a trojan. You have all the time in the world to get infected, but would only take a single time when your files are unencrypted to expose them.
An even bigger problem is Dropbox being the trojan, which is why I disagree with the "ecrypt-fs + dropbox" solution. We live in a world where Internet searches like "dropbox condoleezza rice" and "dropbox prism" are incredibly unflattering.