I get that a few times, but have swapped the 2GB card for a 16GB one. Have cleaned the contacts and the problem has gone away for now, but I am not sure how it might even get dirty/dusty.
It's a right pain if you want to take a photo and have to switch off, clean, boot up and find your photo op is over.
I am wondering if something is causing the SD card to be dismounted in software? Thing is, I can't remember if the last incident was before or after the new firmware.