Nationwide is the poster child of consolidating on zLinux. [1] But of course, that's all marketing from IBM. I believe their target market is to customers who already have the z hardware being used for their z/OS processing. The infrastructure is already there, then customers just pay for the increasing in IFLs.

[1]: http://www-01.ibm.com/software/success/cssdb.nsf/CS/JSTS-7JE...