Being a former US Citizen, Check6 isn't quite right.
If you are a US Citizen and are out of the country over 270 days/year you do get a break (very slight). In the past you started to pay tax on, I beleive it was 72,000 and has been adjusted for inflation. The first 72000 was in essence tax free. There was a MAJOR change a few years ago and now you still have the same exemption but the tax rate starts as if you had paid taxes on the exempted amount. In addition, you are required to include as taxable income any housing assistance you receive. Oh, and in addition , if your host country has taxes you pay them too!
Take a look at this and you'll see what a great deal it is!!!
http://www.irs.gov/pub/irs-pdf/p54.pdf