I don't know of too many commercial lenders that wrote them, but all FHA and VA mortgages since WWII have been assumable, still are today. NOTE: Hmm, seems that's not a blanket statement anymore. Most, but not all FHA and VA loans in the past few years still are assumable but require lender approval. http://www.bankrate.com/finance/mortgages/will-assumable-mortgage-solve-crisis-1.aspx
It very much DOES matter when. If it happens within, say, 12 months, yes, there is a big problem. If it happens 10 years out, it's not much of a problem at all, because the drop is offset by a decade of not paying rent. Japan is starting, what, it's third decade of (essentially) ZIRP - this can go on for a VERY long time. That said, I do agree with the general principle that buying when rates are about as low as they can get is something that requires a little foresight and planning.
Not true. The legislation you're referring to overrode all state legislation that prohibited anything but fixed rate mortgages. I'm pretty sure that Wisconsin was the first state to provide variable rate mortgages in the late 50's or early 60's. But, it wasn't until lenders in California offered ARMs in the early 70's that it started to spread across the country over the rest of the decade.