[rada]

There is a rule - quite reasonable in my opinion - that if you were to renounce your US citizenship but then come back for more than 120 days (per year) during the first 10 years following your "resignation", you are considered a resident of the US for tax purposes. It is designed to deter Americans from giving up citizenship solely for tax evasion purposes, as opposed to actually moving to another country. I imagine one enforcement mechanism would be the denial of entry.