Microsoft’s posted an answer in the Knowledge Base article concerning the use of third-party SD cards in Windows Phone 7 devices… kind of. They reiterate what’s been said before, ‘course:
The SD card slot in your phone is intended to be used only by the Original Equipment Manufacturer (OEM) that built your phone and your Mobile Operator (MO). These partners can add an SD card to this slot to expand the amount of storage on your phone.
However, they seem to strangely conclude the post with a rather teasing (and particularly vague) message:
Determining whether an SD card is Windows Phone 7 compliant is not a simple matter of judging its speed class. Several other factors, such as the number of random read/write operations per second, play a role in determining how well an SD card performs with Windows Phone 7 devices.
What factors, Microsoft, what factors?! Thus, nothing much is truly resolved, in said post. The company still can’t decide whether to officially allow user installation of SD cards, or discard such notion altogether. We’ll hope that things are resolved in the extreme near future.