I think it depends on whether or not the person returns to the United States, if they own property in the US, file US taxes, and so forth. I imagine it is possible if people do it. However, you could get unlucky - if you get a cranky USCIS officer at the point of entry, they can turn you back.

Before we came to Switzerland, we consulted an immigration attorney - my husband is an EU citizen was a Permanent Resident of the US. The attorney advised him apply for US citizenship. Apparently the US doesn't care quite so much where you live if you're a citizen, provided that you file your taxes. We had thought the appropriate thing to do was to give up the green card if we decided to stay here. The attorney told us that even giving up Permanent Residency status is risky - if you want to return to the US having given up your status, they may not want to admit you because they think you're trying to establish residency again. I know this sounds bizarre but there is so much grey in US immigration policy and so much is left up the the USCIS officers at entry points.

At any rate, we filed last year, and he became a citizen in June - we are glad that he's now a citizen, makes it much easier to sell our house in the US, and it makes it easier to travel back and forth.

e.