I've had both, and the older MBP was a very loud fan. The new rMBP is a much quieter fan, but you still hear it without issue. And it won't kick in as easily. It is not silent, by any means. But a bit quieter, but still there, just not as much.
I have one of the last Lapzillas too! I wish Apple would bring them back.
Anyway, my one sounded like a jet turbine. I opened it up and cleaned the fans and vents; there was an incredible amount of dust inside it. That made a big difference ( the fan is less likely to go into jet mode these days).
I agree with both Ben and Simon. The nMBPs fan is very quite in comparison to older models, but it is not "silent" under load. Under normal loads it is hard to hear it.
I have recorded sound using a USB mic in close proximity to the computer without fan noise bleeding in. Something I cannot do with my MacPro.
As Simon says, dust build up will increase the noise and vibration of the fan. I'm about to take my rMBP into Apple to have them clean it, still under extended warranty. I routinely clean my MP every 4 months or so with a major take apart every couple of years, including pulling the CPU coolers. Each time I do this, internal temps go way down and fan speed decreases.
Hope this helps, Greg