I have the same issue like you. bought my one on the 30th may.
I installed gfxCardStatus to check which card the mbp is using. when you plug an external monitor it always switches to the discrete one. I checked as well the fan speed and you could see that an external monitor is attached the fan speed goes up. With gfxCardStatus I tested then discrete one turned on and no screen attached. No issues. Fan speed low and not getting loud.
I had an appointment at the genius bar. we connected as well an external monitor and he confirmed the issue that the fan speed is getting up and it's more loud. in my case it goes max. to 4200 rpm. average is around 3400 rpm. He said he can't do anything. only take it back and try with the mbp internal graphics only.
perhaps it's a software issue from yosemite (I think thats the problem). so either you hope apple releases an software update or it will stay like that. in my view, it could be solved easy. yosemite should use the internal one as well when a screen is attached and only if demand really gets high turn the discrete one on.
because in our offices we have a mbp from the last generation with the iris only and an external monitor (4k screen) is connected, the fan speed is ok and not too getting loud (around 2200 rpm). and cpu and graphics card are the same like this years model.
it's clear if you do gaming and so on that the fan will get loud. but if you just connect an external monitor the mbp just should use the internal card and not turn the discrete one. i'm thinking to give it back and get one with an integrated only. because the iris pro should easy handle an external monitor.
let's see what happens in the next days. perhaps an update will be released.