I had the same problem when my phone was about 3 weeks old. I was using the camera to take a couple short videos (nowhere near filling up the card), when all of the sudden the camera app said "insert SD card".
When I tried accessing the SD card using Astro file manager, it told me that the SD card needed to be formatted. I could no longer access the SD card from the phone or the computer.
I took the phone back to the Verizon store hoping to get a replacement SD card, but they did not have any 16Gb cards at the time. I talked to the store manager, and she suggested that I format the SD card in the phone and if I had any more problems, then she would replace the whole phone.
I have not had any problems with the SD card since formatting it that day, which was about 6 weeks ago. But I make sure I keep a backup of my SD card on my computer now.
So anyway, try formatting your SD card and see if that fixes your problem. Settings --> SD card & phone storage settings --> Format SD card