I've wondered what might happen if IBM lowered costs on this hardware... If they offered a compelling deal, it's conceivable a startup might choose them over Linux. As it stands now, I find it near impossible to imagine any organization starting with a clean slate choosing a mainframe for anything. Cost combined with the work required to make the thing do the things is just way too much of an investment.
Many comments tout the uptime and reliability of the "mainframe", I'd argue we have many counterexamples built on Linux. Building a product with this level of reliability on Linux is expensive but still cheaper than a mainframe and the various support contracts, IMHO.
I started out working with an IBM AS/400 in college and eventually worked for a company that ran their business on one. Eventually market pressure forced that company to move to Windows servers accessed through Citrix from thin clients. In my opinion, this didn't make much material difference: it was still complicated, people on the floor complained about Citrix+Windows just as much as they did the old IBM terminals. Hardware costs and support contracts were less expensive but the cost of the software was much, much more expensive and the organization no longer had the ability to change any substantive functions. Just sayin', moving away from a mainframe isn't necessarily a clear win either.