Larrabee will be scalable to TeraFLOPS, feature a new cache architecture and vector instruction set, and a new vector processing unit / wide SIMD, according to slides provided by Intel. When it comes to APIs, it will have full DirectX and OpenGL support along with a native C/C++ programming mode for all software renderers.
No Intel representatives have publicly commented on the Linux support for Larrabee, but we have received a few bits of information. You can expect Intel's support to continue with documentation and its open-source driver (xf86-video-intel) through Larrabee, but no comment was given whether there will be same-day support for Intel's first discrete PCI-E GPU. Based upon Intel's recent track record though, and with the GMA X4500/X4500HD IGPs being supported before the motherboards began shipping, we'd suspect they'll be delivered in a timely fashion (Intel X4500HD Linux benchmarks). Based upon the ETA for Larrabee and the current rate of the Intel Linux driver's development, perhaps it will be xf86-video-intel v3.0 (the latest release today is v2.4.0).
Fortunately as the first Larrabee products will be delivered once the dust from the the GEM memory manager settles and Gallium3D on its way to reaching a critical mass, it will be an interesting time and should allow for competitive performance. Kernel mode-setting should also be mature within the Linux kernel by then, XvMC supporting more formats, perhaps VA-API, and many other technologies on the Linux graphics front to be excited about within the next year or two. X.Org 7.5 should be out before Larrabee ships and hopefully X.Org 7.6, but based upon the recent X history, who knows?