This thread got me curious, as I had just installed the Picasa uploader a few weeks ago. Sure enough, Google Update Helper was running, and "helpfully" restarted itself every time I logged out and back in. Annoying.
I found
this thread on Google Groups which talks about using
launchctl to disable it, but that solution doesn't actually work. It just kills the running instance, but the app comes back next time you login.
Finally I looked at the differences between my two Time Machine backups, the morning before installing Picasa and the morning after. And it turns out that Google hid the startup of this thing in
/Library/Preferences/loginwindow.plist. Conveniently for them, Apple does not provide a real interface to edit the items in this file. It's the same as the preference file specifying your personal Login Items (System Preferences -> Accounts -> Login Items) but this is system-wide. I'd almost consider filing a bug report with Apple, because users ought to have better control over system-wide startup items.
Anyway, here's how I got rid of it:
- In Finder, go to your computer, Library, Preferences, and double-click loginwindow.plist.
- In the Property List Editor, expand Root and AutoLaunchedApplicationDictionary to reveal a numbered list of dictionaries.
- Expand each numbered dictionary until you find the one that has the Google helper. Highlight that number and hit Delete.
- Save the file somewhere else (it won't let you save it directly due to permissions, so use Save As...) and exit Property List Editor.
- Copy the saved version of the file to /Library/Preferences in Finder, replacing the existing file.
- In Terminal, type sudo chown root:admin /Library/Preferences/loginwindow.plist (this restores the file's proper ownership).
To have the change take effect, you need to kill the loginwindow process. That will log you out, so don't do it if you're in the middle of something, and save any work before doing it.

To do that, just go in Terminal and type
sudo killall loginwindow. Log back in, and no more Google helper.