Despite the downturn of the mortgage market, a type of home loan has remained surprisingly sturdy: one extended to illegal immigrants. Now, the question is whether these loans will continue to hold up. A number of factors -- including a possible government crackdown on illegal workers and a slowdown in job prospects for undocumented laborers -- threaten the ability of these borrowers to keep paying. And there are signs of a slowdown as some lenders have raised the interest rates they charge because of the recent mayhem in the credit markets. Known as ITIN mortgages because applicants must have an...