I think the moment that the water reaches its boiling point, technically whatever you're trying to kill is dead. The reason you're told to boil it is just so there's no guessing if the water is hot enough... If I recall correctly...

I read that in an thread here in the forums. Can anyone back this up, or have I lost my marbles?

In any case, I'd probably let it go for a couple of minutes at a boil because I'm paranoid.

Water must be boiled vigorously for at least a minute to get rid of giardia and cryptosporidium would probably take longer. Most other pathogens (fecal coliform, ameobas, viruses) are destroyed after a shorter period, but if you've ever had a case of giardiasis, as I have, you will gladly boil that water for the full minute, and maybe then some.