I use Mac OS X while logged in as a non-admin user. The problem with this is that the Software Update notification only appears if I am logged in as a user with administrative rights. In a real multi-user environment this makes sense, because the ordinary user should not be confused with things he's not responsible for.
But what about the situation with the typical single user machine, where the owner uses a non-admin account for normal work? (And everybody should do so!) In this case, the user is the administrator, although he or she is using a non-admin account. In this very common case, the user should get the software update notifications so he/she can react to them. However, even if the Check for Updates option is selected in the Software Update panel of System Preferences, there will be no notifications. You can argue if this is a bug or not, but it's how it works.
To solve this problem, I wrote a little AppleScript (in fact, it's embedded into a launchd plist file, so you only have to care about one file) that checks once per day if there are any software updates available. If there are any, they are displayed in a nice looking Growl notification, if Growl is installed (highly recommended!). Otherwise, they show up in a standard system dialog. Here's the code (note that the latest version can be found in this post on my blog):
$ launchctl load ~/Library/LaunchAgents/de.anderson.sven.updateCheck.plist
I've been using this code for over a year now without any problems, so I decided to make it available to everybody.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090227092240386