We have 2 MBP Retina in our household. I purchased mine shortly after it came out, my GF's laptop is just a week old. During heavy CPU usage the fans become noticable. I know this from my laptop only when I do things like encoding videos. However, during normal use of Safari and OUtlook I never ever hear the fans. On my girlfriend's laptop however the fans kick in a few times during the day while she is only using safari. One might say that she looks at flash heavy sites, which is not true. She was just reading BBC News like ma at the same time, and I could hear her fans spinning. The CPU usage on her laptop was around 5%.
Not sure why this only happens on hers. I have a ton of stuff running all the time, and I never hear the fans spinning in my MBP.