I think I remember reading a while ago (not sure where I read this or if it was a reliable source) that a US citizen settled out of the US can be detained or have their passport confiscated for large unpaid debts if they come back into the US, even for a short visit. I was discussing this with a fellow ex-pat here in the UK the other day, who is considering just not paying her student loans for a while since she was made redundant late last year and she and her husband are really struggling - so if she defaults on her loan and comes back to the US to see her family, can she be kept in the country against her will? If so, for how long? Until the entire debt is paid?
Original text: Can your passport be confiscated for unpaid debt?