Same thing happened to my Droid X around May 4th or 5th. I suspect an app update rendered the microSD card unreadable.
I fixed it by removing the microSD card, plug it into your PC (you will need a microSD to standard SD adapter so it can fit in the SD slot on your PC), copy the pictures to your computer hard drive, then format the microSD card. Once done, copy the pictures back to your microSD card.
Now try the microSD card in your phone... it should work.
Good luck. Reply back if it works.