Since there can be important security updates and a lot of people don't check the site, the Bitcoin client should have an optional auto-updater (on by default), with "how often?" options ranging from each five minutes to each day and an option to install without asking (only security updates or all updates?)
I can see this as a security risk if the updater were able to be set to automatic. Invariablely, some users will disregard the risks in the ongoing absolute trust of a particular server, and enough might be able to break the system if some cracker were to be able to compromise that trusted server and replace the client download with a compromised client with malware. Even if that only lasted for a short time. If the client were to ever include an update notification function, I disagree that it should *ever* update without user verification. Even a normal client modified to send a copy of your wallet.dat file to a particular email address would screw a lot of people over in a hurry.