I certainly can't see how 40 year loans would be better than IOs for investors (unless the interest rate for the 40 year loan is lower than the principal repayment making payments on the 40 year loan lower than the IO loan). Even if the principal repayment component is lower on the 40 year loan, there's still SOME repayment, and as investors we would want to minimise that.
Reading the US newspapers, they're saying how ARMs (adjustable rate mortgages, i.e. floating rate mortgages) and interest only mortgages means a lot of people are going to get hurt from the property downturn. Traditionally the US used 30 year FIXED RATE P&I loans!!! Here in Oz, we've been using floating rates and IOs for longer. On the other hand, PPOR interest is deductible in the US, which makes buying your PPOR much more compelling (and is part of the reason why yields in the US are so much higher than in Oz, because there are less 'investors' who buy for the 'tax breaks', since in the US your PPOR IS a tax break.)
Seems to be just the market developing new products as it goes along. I honestly don't think it'll change the general trend of booms and busts in the markets. Long term, it all evens out. As investors, we should still buy when we are able to, within our own safety criteria.
Alex