Bitcoin QT, being a Linux application, is updated regularly when the application in the repository is being updated (For example, apt-get upgrade on Debian).
Theoretically, if a hacker could get into some Linux repository, he could replace the application with a malicious one that steals both the private key and the wallet file and send it to a remote server (effectively stealing all the Bitcoins from most the users). If possible, even through social engineering, the incentive for such hack is enormous.
How likely/unlikely that to happen?