Such a small set of testers especially considering how many of them will go to friends/industry probably means they are expecting to get buried under a shit storm of bugs.
With such a wide range of kernel versions, OS differences and various flavours of drivers from open, proprietary and closed sources, yep it's going to take some work.
From the post, it looks like they are only supporting Ubuntu. Not that that will stop it being repackaged for other distributions when it's released. From the collaboration with Intel, it will be interesting to see how much attention they pay to the other drivers.
Even just supporting 1 version of Ubuntu, you will have to support a wide range of kernels and configurations. I'd say that using a custom kernel isn't too far out of the norm.
The kernel won't matter. The hardest part with compatibility is library versions (for matching API and ABI), and the opengl rendering of the graphics driver (which Valve is working with Intel on).
