Is x86 important?
I'm not sure x86 compatability is important. HPC codes are not written directly in x86 assembler and to a very large extent are re-compiled for each system rather than being distributed as binaries.
Historically the HPC market has been happy to migrate to whichever architecture provides the most value for money. The current dominance of x86 chips in HPC is more due to the price performance benefits diven by the mass market in x86 chips than by any requirement for x86 compatibility from the HPC market. This would argue that graphics chips have the advantage due to selling in two markets (unless of course intel subsidise their HPC only chip).
Programmability is an issue but the PGI compiler already targets nvidia GPUs to accelerate normal fortran codes. There are problems with targetting attached co-processors with their own memory but these would also be experienced if the attached processors are x86 cores.
Of course x86 compatibility does mean you might be able to build a large MPP system out of ONLY Larrabee chips which might save some money.