> Don't forget that IBM supports running Linux on all that nice Power hardware as well.
Yep, but I suspect that the number is relatively small. I have never seen any estimates, but given that RHEL as well as SLES is available on Power it must be worth it. There certainly is Linux on Power, i.e. Watson, the system that beat the reigning Jeopardy champion ran on 10 racks of Linux/PPC . There is also a lot of Linux in HPC, i.e. Blue Gene with the third generation Blue Waters coming  (aka Blue Gene/Q in marketing terms). Even though none of the three generations is not really a 100% Linux system since only the IO nodes run Linux while the compute nodes have some minimalistic kernel .
IDC gives estimates for operating system market share in the server market (see  for the second quarter 2012) and while all those reports should be taken with a pinch of salt (or discarded like the insane Itanium projections from the late 90s :), these do seem to make sense.
I quoted my numbers above from memory and it seems AIX is probably slightly less than five billion and the Linux hardware market clocks in a little higher at around ten billion a year. Ironically systems like the Sparc based Fujitsu K (HPC 500 current second most powerful system) also count there since it does run Linux and not Solaris. All the other deployments of those Fujitsu HPC systems I have read about also run Linux even though it is also offered with Solaris. That should tell us something about the HPC market and the demand for Solaris in that sector ;).
> I'm sure that many IBM shops have so much invested in AIX software that they don't take much advantage of it, but it's an option.
Yep, with virtualization there is the possibility to run mixed workloads and given how much effort IBM spends on KVM/PPC it seems that there is demand. Linux on S390 certainly seemed to have helped IBM in the main frame business.