Alexander von Gluck of the decade-old Haiku OS project has approached the Mesa developers about integrating Haiku OS support into mainline Mesa. Haiku OS right now uses Mesa, except it's a heavily-modified version of Mesa 7.4.4. Mesa 7.4.4 was released in mid-2009.
Haiku OS developers know regardless that upgrading from Mesa 7.4 to 7.11/7.12/8.0 is going to be a massive pain, but they're hoping to get Haiku-specific patches integrated into Mesa so that it will be less of a pain staying up-to-date in moving forward. Haiku support patches were brought up in this mailing list message.
Brian Paul already responded saying he's fine with seeing Haiku OS support patches assuming they're added cleanly like the other OS-specific patches. While the Mesa OpenGL library is most commonly talked about for Linux and that's where it's full-featured, Mesa is also used on Solaris, *BSD, Android, and even Windows/Darwin/MacOSX, among other obscure platforms. Mesa/Gallium3D was even ported to AROS, the interesting research OS last year.
Past Mesa efforts involving Haiku have been offering a bounty ($2,000 USD) to port Gallium3D (and related dependencies like GEM/TTM memory management) to the BeOS-successor, preliminary Gallium3D work, and hopes for a new 3D driver stack through a Google Summer of Code effort but that never materialized.
In a follow-up message to the Mesa developers, Alexander von Gluck mentioned that Haiku OS is mostly interested in Radeon HD and Intel graphics support at the moment, but not any of Mesa's legacy drivers. The Haiku developers have already done some work with a kernel-space RadeonHD graphics driver and a user-space graphics "accelerant" that maps graphics driver memory, and then a user-space OpenGL/3D renderer that taps the accelerant.
It will be interesting to watch how the Haiku OS project handles their Mesa upbringing -- especially seeing as even the BSD and Solaris graphics drivers are in a very poor state.