I (and many other "tin-foil-hatters") do not like nor will use software that will automatically update without our consent, and fully-automatic updating does not look good security-wise.
I second this ... highly...
It'd be a security risk.
I would like a popup that alarms the user of any updates though.
Like "Bitcoin has updates! We suggest you update as soon as possible, go to
http://bitcoin.org/ to get [version]." or something.