But when the balance hits a certain level, payments often jump significantly, and borrowers can run into major financial trouble.

FirstFed potentially faces darker days than peers who play in the same niche. For one thing, all of FirstFed's mortgages are for homes in California, where prices have cratered and foreclosures have skyrocketed. Also, 80% of its loans have little or no documentation to prove the borrower's income or assets, according to a recent company presentation. The bank uses credit scores to screen for elite borrowers.

But skeptics are starting to question the quality of FirstFed's earnings. The bulk of FirstFed's income is derived from noncash earnings, largely from the deferred principal on its option ARMs. That so-called negative amortization constituted $223.9 million, or 68.4%, of the bank's income before taxes in 2006, compared with 1.3% in 2004. In essence, FirstFed is booking profits on money it hasn't collected. The fear is that the bank will never collect, given the high delinquency and foreclosure rates in California. Says Frederick Cannon, managing director at Keefe, Bruyette & Woods Inc.: "The bearish view is that all the earnings are coming from money they didn't get yet."

FirstFed admits the environment is tougher today, but says its borrowers have stellar credit and can afford to keep up with the option ARMs' rising payments. Indeed, FirstFed's loan portfolio, with a higher credit quality and lower delinquency rate, is holding up better than those of larger competitors such as Countrywide Financial (CFC ) in Calabasas, Calif., and Washington Mutual (WM ) in Seattle. FirstFed CFO Douglas J. Goddard says the bank fully expects to collect on its loans. "In our nearly 25 years of offering this product, we have yet to find where payment shock' caused a default," he explains.

Still, given all the red flags, it's no wonder short-sellers have pounced. Some 40% of the company's 15.3 million shares have been sold short. That dynamic may have helped boost the stock. As it climbs, hedge funds and others rush to buy more shares to cover their money-losing short position, pushing the stock higher.

And as it goes up, the stock is attracting new buyers. "I constantly see momentum players buy financial companies because they hit some screen, but they don't really know what they own," says Richard Eckert, senior research analyst at ROTH Capital Partners in Newport Beach, Calif. "They are not distinguishing between cheap, and cheap for a reason."With borrowers sporting higher credit scores, FirstFed Financial does not appear to be a sub-prime lender in the traditional sense - at least not yet.

Apparently catering to the market for which stated income and negative amortization loans were originally intended - small business owners or independent contractors with large but hard-to-document and/or unpredictable income - the company is faring better than most sub-prime lenders today.

Tomorrow, however, is an entirely different matter.

Their clientele likely includes the professional real estate speculator crowd that was in early on the California real estate craze - some of them have clearly lingered too long.

Not to be confused with your typical equity-rich Southern California homeowner who took the plunge with investment property in 2004 or 2005 after being convinced that real estate only goes up, this group of borrowers has probably been in the game for many years now and has done quite well.

Until now. Just like the craps tables at Vegas, it's hard to know when to stop.

While FirstFed Financial may not currently be a sub-prime lender, with four times the number of problem loans as a year ago sure to impact their customers' credit score, they may turn out to be one after-the-fact.

Full disclosure: No position in FED at time of writing.