Reset the camera app ... (This happened to me once, and resetting the app worked)
SETTINGS > APPLICATIONS > MANAGE APPLICATIONS > ALL > CAMERA > Force Stop (if necessary) & Clear Data
After I did this, my camera wouldn't force close when switching to the front camera.
Downside(s): Camera settings are reset, so if you've turned off the sound, for example, you'll have to turn it off again.
Don't worry, your pictures are safe.
Hope this helps.