I've seen several people having similar problems, but wanted to put my specific situation out here to see if anybody else is seeing it.
I bought a Droid Pro in November of 2010. Not long after I had it, I turned it on and it gave me the "SD Card is Blank or has an Unsupported Filesystem" message and forced me to format it. I was able to format it through the phone and it worked for another few weeks when the phone did a hard lock up and I had to remove the battery...it did it again. Anyway, it happened to me again last night (hard lock up, battery removal) and now I can't even reformat the card through the phone. I put the card in the adapter and put it into my laptop and the laptop doesn't recognize the card at all....it's junk.
My wife has a Droid X and she's encountering the same problem. Her Droid X also has an issue where she takes a picture but it won't save. You'll see the thumbnail of the just taken picture on the camera screen, but when you click on it to open the gallery, it's not there.
We're both using Sandisk 8gb Micro SDHC cards. Could it be the brand? I've just reinstalled the stock 2GB card that came with the phone and it recognized it.
The only aftermarket apps I'm running that are interacting with the SD card are Doubletwist and iSyncR. I've lost about $40-$50 in music because of this and have now stopped downloading anything onto my phone making it fairly useless to me as I'm too afraid to buy music or take pictures with it because I fear that I'll lose them. Verizon swears they've never heard of this issue. They offered me another Droid phone but the only other one I would get would be the Droid X and as I mentioned, my Wife is having the same problem.