This exact thing happened to my Eris after the OTA update was installed the other night, which I just discovered. I'm VERY mad, as I lost some precious pictures of my 1-year-old niece, and some pictures I had taken around my parents' farm that are irreplaceable. The music can be restored, big whoop there, but the pictures are forever gone.
Not only that, but when I tried to mount the card to Windows via the USB cable, the phone suddenly decided that it didn't like the SD card anymore, and said it was damaged an unreadable. I couldn't even format the dang thing! I had to pull the card out and put it in an SD card reader, and then Windows could read it and format it. I then put it back in the phone and it mounted okay, but everything is definitely gone now. That's the last time I will EVER install an OS update without backing up my data. I and really, really mad because I have never, ever, ever, EVER seen ANY type of OS upgrade on any computing device wipe out user data, unless hardware failure was somehow involved. I work on computers for a living, so I've seen a lot, but never saw this coming.
The cruel irony here is that all the data on the internal storage is still intact. Pictures for things like backgrounds, contacts, etc. is all stored on internal memory, so that's all still there, but that's it. I'm now trying to figure out how I can retrieve the image I have set on my lock screen from the phone's internal memory, as that's the only photo of my niece I had on the phone that's still intact. If anybody can tell me how to do this, I would be most grateful!