A couple of days ago Intel's Pat Gelsinger told the press that the upcoming Larrabee GPU is based on a bunch of P54C Pentium cores and today ARS Technica offers a bit more background information. The site says Intel gave the RTL code for the Pentium to the Pentagon quite some time ago for use in military applications. A few years later the Pentagon offered the RTL back to Intel and according to ARS the Larrabee will be based on this "Pentagon Pentium" architecture:
Quite some time ago, after the Pentium was obsolete and Intel had moved on, the company gave the RTL code for the processor to the Pentagon so that the military could continue to fabricate a radiation hardened version of it for use in military applications. Trailing-edge hardware like the Pentium has the advantage of having been thoroughly tested and debugged (cf. the P54C's infamous FDIV bug), and at the time the military had its own fab facilities that could do some low-volume fabrication. (I'm sure they still have such facilities for prototyping.) So the Pentagon cleaned up the P54C's RTL code and began producing a rad-hard version of the chip for military use.
A few years later, when the Pentagon had moved on from the P54C, they offered the RTL back to Intel. So Intel took the core, which has a very small footprint and by this time had been pretty thoroughly debugged, and modified it for use in the many-core chip that later became Larrabee.
The reporter also heard Intel will claim Larrabee has 20x the performance per Watt of a Core 2 Duo and half the single-threaded performance. The GPU also has 4MB coherent L2, and three-operand vector instructions. More information may be shared by Intel later this summer.