Microsoft has previously mentioned that Windows Phone 8 will allow users to install apps from SD cards. The claim's pretty funny. Who needs to install apps from a SD card anyway? We've got the Marketplace, or whatever it's renamed now, and that makes a fine enough source of applications. If anything, we will need the ability to install apps to the SD card, to better utilize the expanded storage property.
But it looks like Microsoft has been serious about it. Here in the company's online Windows Phone 8 FAQ, there's a little chapter instructing you on how to just to that, installing an app in the form of a .xap package from your SD card to the phone.