Palmer has a bit on that in his book, suggesting that you do not need to extract all of the sugars. The goal is to make the best beer you can out of the ingredients, not the maximum amount.
That being said, there is no reason not to add hops throughout a 120 minute boil, at least for an IPA...
Jtakacs, I actually just did the same last weekend. Citra Pale (og 1065), US05, to make the yeast cake. Then the bw.
But when I left my house yesterday (on the road now) my originally 1100 wort was still gently bubbling through the blow-off. So mine is taking longer. Aerated (pump with stone)...