Same thing here.
One of my old sites was in top 10 for all main keywords. In the middle of March, shot in the head for all keywords and got a -50 to -80 positions across the board. Site has been there ever since. No sign of growth. No nothing. Site was a store with over 2000 products and unique descriptions for all products, as well as a blog with about a thousand posts, all unique and on topic.
Also just recently in April had another site rise to top 10 for my targetted keywords. Got stuck in position 8 and decided to do some on-site optimization with IBP. Within 24 hours I was gone out of the listings completely, and only a week later the site returned to the 90's positions. Also a huge store with thousands of products.
Google has been nothing but a nightmare for SEO after the Panda update.