There used to be a special "expatriation tax" that applied only to taxpayers who renounced their (tax) citizenship for tax avoidance purposes. However, under current law, I believe you are treated the same regardless of your reason for renouncing your (tax) citizenship. Here's an IRS page on the subject:

http://www.irs.gov/businesses/small/international/article/0,,id=97245,00.html

This is not an area of my expertise, though.