Marty,
I'm running WAS on Sun V880 and 280R servers with Solaris 8. We are just beginning to explore WAS running under Linux on a z/OS platform with zVM. While I'm still in a WAS shop, we are looking at a need to distribute servers to a variety of locations, which begs for a lower cost solution than WAS (perhaps JBoss on Linux).
Sorry I can't be of any help with the OS/390 or z/OS, but if we get there in the near future, I'll let you know what we find out.