There was a lot of confusion about Windows Phone 7's microSD card support approaching the launch. Most phones do not have a slot, and some of those that do are not accessible. One notable exception is the Samsung Focus on AT&T, which gives users easy access to the card. Now Engadget has gotten word that AT&T is recommending that people not try to swap in a bigger card, or risk degrading performance.
The problem is in the way Windows Phone 7 uses that extra space. It is merged with the internal NAND flash memory to create one data store. So a card that isn't up to snuff could bring things to a messy halt. AT&T says that cards will be available in the future that are "Certified for Windows Phone 7". They have also cautioned that the speed class rating is not the only determining factor.
Microsoft was aware of this issue prior to the launch, but for some reason AT&T is only getting around to telling customers now. Do you have a Samsung Focus? Have you successfully swapped the microSD card?