I’m not aware of any solutions for iterating through and automatically updating your list of plug-ins, but my guess is that any system that can automatically update WordPress itself can be modified to update everything in fairly short order.
That said, I know of two good ways to update WordPress automatically:
WP Remote
This is an online service that allows you to remotely monitor and update WordPress when needed. From the initial description I received (I haven’t had the opportunity to test it myself), you can set the system to update things automatically without your intervention.
WordPress Automatic Upgrade
I have several clients who swear by this plug-in. It will automatically upgrade WordPress (not your plug-ins) whenever an update becomes available.
Update 7/13/11
Another option, since you’re already using cron to schedule the update, is to script it and use Subversion.
If you install WordPress via Subversion, you can just run svn up to automatically update to the latest version. This will update your files, but not the database … so you’ll need to script a hit to the code that runs the db update as well.
Plugins will work the same way. Rather than using the built-in installer, check out the plugins you’re using via Subversion and iterate through the /plugins folder with an svn up command to keep them up to date.
Some of my sites are using private plugins that are hosted on GitHub – I use the same process there. I check out the plugin using git clone and just run git pull on a cron to keep them up to date with the latest version.