To take the logic just a little further: how long will interest rates and other rates of return (stock market returns, certificates of deposit, etc.) remain as low as they are today. Six months? One year? Two years? If you think you can make 5% or more annually through some other investment, you should consider maximizing your mortgage and investing the cash. Risk of loss is something else to consider: a Baby Boomer with retirement in a few years will want to steer away from riskier investments, but the point is still the same. The totally risk-averse person should just go ahead and pay down the mortgage - which is a nearly risk-free return equal to the mortgage rate.
Locking into a generally long-term mortgage at today's ridiculouslly low rates is a very good thing for several reasons. First of all is the obvious lower interest rates. Second, this will generally provide the lowest minimum payment. A lower minimum payment maximizes your personal liquidity and ability to deal with unexpected problems without the threat of defaulting or messing up your credit scores. Third, you can almost always make additional principal payments.
Certainly true if you currently own a home which you are going to stay in for a long time. I personally don't think it's the best time to be buying a home, though, since the low interest rates have pushed up home prices considerably. IMHO, it's probably in many ways better to have a smaller loan at a higher interest rate than a larger loan at a lower rate, especially if the higher loan has a very high LTV. If someone buys a home now and interest rates go up (forcing home prices down), they may be able to afford the fixed monthly payments but may be quite unable to afford to sell the house (since the falling market would kill their equity, and any replacement house would have to be bought at a higher interest rate).
A lot of computer programmers and others who are out of work today would not have anticipated they'd be out of work 2 years ago. I think liquidity is very important now ---if you have to foreclose on a mortgage because you lose your job in one or two years, then it would be best not to have so much into it. Also if you can put the money into savings, you might not have to lose the house. I'm thinking of going the other way if I refinance which I might if the economy worsens so I'd have lower payments and more flexibility.
Actually, the return effectively exceeds the mortgage rate, as money not earned cannot be taxed.