Engadget's take.
While running Diablo:
"Still, it only took a few minutes of hacking and slashing to get the bottom of this unit warm, and then noticeably hot. That, of course, caused the redesigned fan system to pop on, which draws in air from a pair of vents on the left and right sides of the bottom of the chassis and blows it out through the hinge. It's been optimized to create a less obnoxious sort of whirring noise. Indeed it's a subtle and unobtrusive white kind of sound, but it's definitely not silent. In fact, the fan doesn't sound particularly different than that on the current MacBook Air, though a few decibels less obtrusive. Still, you'll always know when your system is really cranking."
Still, it only took a few minutes of hacking and slashing to get the bottom of this unit warm, and then noticeably hot. That, of course, caused the redesigned fan system to pop on, which draws in air from a pair of vents on the left and right sides of the bottom of the chassis and blows it out through the hinge. It's been optimized to create a less obnoxious sort of whirring noise. Indeed it's a subtle and unobtrusive white kind of sound, but it's definitely not silent. In fact, the fan doesn't sound particularly different than that on the current MacBook Air, though a few decibels less obtrusive. Still, you'll always know when your system is really cranking.
Slashgear:
"Cooling has become something of an obsession among Apple’s engineers, and the new MacBook Pro is evidence of a new strategy for both quiet and effective heat dissipation. Air is sucked in through the hinge section and then funneled through to gills on the sides of the notebook, driven by a newly-designed asymmetric fan with unevenly-spaced impeller blades. That unusual blade design, Apple says, helps to reduce the tonal impact when the fans are spinning.
In practice, it’s a different type of noise to before: not necessarily quieter, but less intrusive. You still hear the fan spool up when doing heavy-duty processing, such as video exports, and the base can become warm – though not hot – to the touch at those times, but it cools again quickly."
Time:"Some fast laptops tax their processors to such a degree that they heat up like pavement in the summer, forcing the use of noisy fans to bring the temperature down. The famously fan-phobic Apple says that it tuned the ones in the Pro to run at different frequencies so the whirrrrrrrrrrrr is less obvious. Even when I loaded gazillions of browser tabs, streamed videos and ran a virtualized copy of Windows 7 courtesy of Parallels Desktop, the Pro kept its cool and I couldn’t tell if the fans were active without pressing my ear to the case."