Re: "ARM's larger and healthier software ecosystem?"
"until a couple of years ago, Linux on ARM was pretty poor."
Maybe the household names didn't have ARM support, but in the small and not-so-small embedded systems Linux world there were plenty of Linuxes (commercially supported and otherwise), some of them more than capable of providing all you need for a compute-oriented node. I used to use MontaVista simply because that's what the hardware/box vendor supported, but other options were readily available.
Whereas neither Intel nor SPARC had (or probably will have in the foreseeable future) any significant presence in the compute-node market. Intel's NextUnitofComputing , or this quarter's hot SoC from an ARM partner? It's not really a difficult decision for most people even if Windows is currently a pre-requisite (which is a bit rare in the HPC world, despite several years of MS trying to strongarm people into using Compute Cluster Server 2003 and then Windows HPC Server 2008 and friends).