Right now it's at least successfully building and hitting run-level five on an Apple MacBook running Debian GNU/Linux both on the hardware itself and virtualized. The kernel that this is being done with is Linux 2.6.36 using the x86_64 architecture with SMP. This build can also self-host itself with the developer reporting he's running a fourth-generation self-hosted Linux kernel built by LLVM/Clang and that the compiler build is also onto its fourth-generation build.
While kernel sub-systems like NUMA, SMP, the IPv4 networking stack, graphics drivers, and other drivers/firmware are being built, there still though are some things being left out of this Clang Linux kernel build. Some of the items not building right now are SELinux, POSIX ACLs, eCrypt, IPv6, anything using the crypto API, and Xen virtualization. The other huge caveat right now to this Linux kernel built by CLang is that kernel module loading is completely broken.
Bryce Lelbach, the developer announcing this work, is hoping to have the kernel in a much more Clang-friendly state in the coming months to the point that it could be considered production quality if using his Git repository that will contain these patches for Clang building.
The announcement of achieving this feat can be found in this email.