Mortgage rates have plunged suddenly -- but not because of any action by the Federal Reserve.

Instead, it was the federal government's decision to take over struggling mortgage giants Fannie Mae and Freddie Mac that sent rates down.

Thanks in part to these lower mortgage rates, first-time homebuyers who scrape together a substantial down payment may be looking at the buying opportunity of a lifetime, says Bob Walters, Quicken Loans chief economist.

"(For) first-time homebuyers, it's the most incredible opportunity in the world," Walters says.

Lower mortgage rates and falling home prices are also a boon to many homeowners looking to trade up to a bigger house.

However, homeowners who can't sell their homes or who are upside down on their mortgages will not be able to take advantage of the opportunity, Walters says.

Meanwhile, tightening credit conditions may prevent many home shoppers -- and homeowners looking to refinance -- from taking advantage of lower rates, Walters says.

"The losers are the people who are upside down on their homes, the people who can't refinance … because they don't have equity in their home anymore," Walters says. "Those are the folks who are struggling."

Thinking about buying your first home? Falling mortgage rates and plunging home prices make now an ideal time to take the plunge, Walters says.

"Given that a 30-year mortgage rate is now in a high 5 percent or low 6 percent range, it's all coming together for a first-time homebuyer," he says.

Homeowners whose mortgages are about to reset -- whom Walters characterizes as being on the "cusp" of potential economic difficulty -- or who have a high mortgage rate -- should take advantage of any opportunity to refinance now.

"Credit tightening is continuing and underwriting guidelines are continuing (to tighten)," Walters says. "So folks who can refinance should, especially people who are on the cusp."