Yesterday, the 16GB microSD card in my Droid X stopped working. It's dead. I plugged it into my computer using several different adapters and the computer did not recognize the card. There was no way for me to reformat the card or do anything with it. It's a brick.
Fortunately, Verizon Wireless was kind enough to issue a replacement card through their warranty program. The technical support team did comment that some Verizon Wireless customers noticed problems with their microSD cards when they upgraded Android from 2.1 to 2.2 (Froyo).