Although we're of the camp that doesn't mind that the Windows Phone Marketplace for apps has been retired from Zune Desktop, we imagine some of you think otherwise.
Luckily, Windows Phone guru Den Delimarsky has you covered. He figured out exactly what the change was that Microsoft pushed on to us all yesterday. More importantly, he also figured out to block that change so you can get back the Marketplace for those apps.
Unfortunately the change is a little tricky. Evidently the "update" comes from a simple modification in the configuration.xml that is sent from Microsoft every time you launch Zune Desktop. All you need to do is switch a "disabled" setting to "enabled" but the tough part is you have to intercept that .xml file.
That's where you'll need a mini-server (or something analogous). Basically you redirect your Zune Desktop to a local server where you can have it fetch your modified .xml file and boom, you're in business. Now of course, this is only works so long as you stay with version 4.8. If you update the Desktop client (and Microsoft is sure to push one eventually) you can probably kiss this trick goodbye.
Anyway, cool stuff just remember, you're using this at your own risk (sorry Microsoft if you get mad!). Go read the whole thing at Den's site for all the details.