I made a grapefruit ginger blonde at the beginning of summer that came out really well. I used grapefruit zest as a 5 minute addition in the boil and more zest (and fresh grated ginger) as a whirlpool addition. I cooled the wort to 170*, added the grapefruit zest (and ginger) and held it there for 20 minutes. I also added the juice from the grapefruit (18 oz) after fermentation was complete. For the record, the gravity of my grapefruit juice was 1.054.
When I do it again, I'll omit the zest in the boil. When I added it, I think it just vaporized the oil in the zest. I don't have any way to scientifically prove this, but it boiled and splattered as if you just dripped water into hot oil.
During fermentation and gravity samples, I noted that there was a huge grapefruit nose from the zest, but barely any flavor until after adding the juice from the grapefruits.