Well some of this is my own speculation on the matter:
The R500 IIRC was a project that was initially meant for replacing the R400 in the PC space (which by itself was a rather small evolution over R300, meaning ATi's resources had been centered elsewhere), but was put aside circa 2004. Supposedly, back then the transition to unified shaders and an effective use of eDRAM in Windows could be a huge hassle driver-wise, so instead they sold the VEC4+Scalar architecture to Microsoft, who was looking for something with a lot of potential inside a closed environment. Then R520 and R580 came with non-unified shaders and by late 2006 they figured Terascale (VLIW5) would be more appropriate for the desktop.
Xenos' VEC4+Scalar was still used for ATi's Imageon line of GPUs for handhelds (back then almost exclusively used by Intel for their ARM-based SoCs called XScale), specifically for their next-generation OpenGL ES 2.0 Imageon called Z430, or Mini-Xenos. Qualcomm used ATi's Imageon for their MSM7xxx SoCs and the Z430 went into MSM7227 and the first-ever mobile SoC with a 1GHz CPU called Snapdragon.
When Qualcomm bought the handheld GPU division from ATi (by then already a part of AMD IIRC), they changed the Z430 name to "Adreno", which is actually an anagram of "Radeon".