The rapid growth in popularity of interest-only mortgages is having a noticeable impact on the prepayment characteristics of fixed-rate loans and will likely become a permanent factor in the market, according to a new report by Bear Stearns. Interest-only loans were introduced first as a financial management tool for wealthier borrowers and they became widely available in the jumbo market in 2002, the report notes. Their popularity grew quickly and in the middle of last year