Sand - 12:31 pm on Jun 23, 2012 (gmt 0)
I don't know why this is happening, but I can offer a few ideas for the sake of discussion.
1. Not everyone is seeing the same results that you are. Have you tried checking rankings through a proxy or third party tool? Or asked a friend in a different part of the country to check?
2. Is there any seasonality around your niche that could be bringing you down right now?
3. Were you hit by Penguin or Panda? I've seen very clear evidence of traffic throttling impacting affected sites.
4. Has the layout of the SERP changed? IE: more ads, local results, videos, images, shopping results, etc?