Define scalability please. If we are talking about single system image scaling up, such as a SGI Altix monster, nothing but Linux will run on it. I'm sure ill be proven wrong, but I'm unaware of a modern general purpose operating system that runs on > 2048 CPU beasts other than Linux. Certainly not Solaris. If we are talking about scaling out horizontally, Linux is the defacto tool for most supercomputer clusters in the top 500 supercomputer list. Heck, solid SMP is still a relatively new thing compared to Linux. When I see "scalability" touted as a feature any of the other 'nix or bsds have over Linux, I really question the other person's definition of scalability. It likely is not the same as mine.