It was a complete oversight of my own, which I thought was a complete oversight of the TYPO3 development team, but for days, I saw no apparent way of updating the local extension list from the remote repository

I've installed the TYPO3 6.0 Introduction Package, by the way.

After clicking on the "Get Extensions" tab of the Extension Manager, I saw no way of updating the extension list.

Then, by chance, I clicked on some text that read "last update: 2012-8-29 (0 extensions)". 

(The date in your install may differ).

Once I clicked on that, my TYPO3 6.0 install started downloading the extension list! Hurrah!

So, basically, all you have to do to update the extension list from the remote repository is to click on that text that says "last update". That's it!