It isn't just the French who are renouncing their citizenship:
http://www.telegraph.co.uk/expat/990...-tax-rise.html
I'm not quite sure how becoming British helps with lowering taxes, but one line stood out.
If an American sells a house in England and has capital gain, he must pay capital gains tax to the IRS, but NOT to the British Inland Revenue.
So maybe Americans selling properties for big gains are dumping US citizenship in order avoid capital gains taxes.