A DMM will give an indication of a drastic fault, but the potential applied (typically 1.5 to 9V) isn't nearly high enough to give an accurate guide to the insulation's condition.
In the U.K., on our standard 1-ph and 3-ph (240/415V) systems we test at 500V and a result of 1 Meg or greater is required to satisfy "code."
As has been mentioned already though, there's more to it than just "over 1 meg good, under 1 meg bad."
If I tested at just over 1 meg on an old heating element, for example, then I'd be satisfied that I was reading normal leakage due to the age of the element. But if I tested even as low as 5 meg on newly installed cables I'd be wondering if I'd damaged them somewhere during installation.
