This may seem a deathly boring (pun intended) subject but since I don't have many readers to bore in the first place and BW specifically asked anyway, here it is.
I finally managed to track down some specific information, with examples, about the how the new threshold is calculated when unused inheritance tax exemptions are passed from one spouse (or civil partner - can't be seen to be discriminatory, can I?) to the other.
Essentially, what you get is the unused proportion of your spouse's exemption, multipled by the current exemption, which is then added on to your own.
So, regardless of the fact that the exemption was only £64,000 when my grandfather died, since he left all of it to my grandmother, 100% of the exemption was unused and therefore, at today's rates, his exemption is worth the full £312,000. And since her estate is worth nowhere near £624,000, we won't be paying Mr Darling anything at all.
If, on the other hand, he had left £32,000 to my mother when he died then he would have used up half of his exemption and the unused part would now be worth £156,000.
Simple, really. I just wish it was a little more obvious in all the commentary written about it when the change was announced.