The Nouveau DRM driver hasn't enabled the fan management support by default from the get-go out of fear of issues with the reverse-engineered support. If fan management goes awry, the GPU could potentially overheat and damage the hardware. So out of caution, even if you have a high-end NVIDIA GeForce GPU with a monstrous cooler, it may sound like a jet engine to be on the side of caution. With dynamic fan management, the fan would spin down under minimal load and then ramp back up under heavy GPU load or when the GPU temperature exceeds a certain threshold.
The support has been present in a few kernel releases now and the testing exposure it's had seems to be positive. Martin Peres (a.k.a. MùPùF of the Nouveau project), wrote in the forums, "Support for fan management has been available since December 2012 but we haven't activated it by default because some bugs needed to be ironed out. Linux 3.12 will bring some fixes for that. I am discussing with the maintainer the possibility of enabling automatic fan management by default in Linux 3.12."
So while it's not yet enabled by default and there was already the lackluster Nouveau changes for Linux 3.12, there might at least be some effort to get dynamic fan management enabled by default for this next Linux kernel release.
When asked about the missing re-clocking support for modern (NVIDIA GeForce Fermi/Kepler) hardware, Martin Peres responded, "This is a huge feature and doing one thing slightly wrong and it will be unstable. This isn't something easy to debug... My laptop would also greatly appreciate some nouveau PM!"
For those wanting to try out Nouveau fan management on their systems and report the results, see the Linux kernel documentation.