I have less faith in the Nissan dealer now, the mechanic they had was deservedly promoted and now they've got a new apprentice. Without performing the fluid couple test, and for normal driving, my coolant system was fine. I agree that they should have figured out the fluid couple was shot, but once I had it diagnosed by Burtons they replaced it for me.
Burtons were spot on, once we changed the fluid couple my car handled heat much better. That's how I now know to check it, and why I'm now just buying another fluid couple instead of mucking around with the dealer.
I'm not stopping with the fluid couple work though. I'm looking at the radiator thinking to myself that I could replace it with something that has better cooling performance, but I'm not quite sure that going to one of the cheap all-metal jobs from eBay is the way to go.