some interesting reasoning to be had here.
after reading through all the arguments previously mentioned, it still appears that the bigger IT shops with the bigger budgets will be able to get value from this technology much more easily, if only because of the complexity and administrative overhead required for virtualization fail-over relationships between various multi-core boxes, along with maintaining the higher-end storage and server hardware, day-to-day operations, updates, upgrades and projects.
those bigger shops may be better served by a mainframe running several virtualized environments (IBM's LPARs come readily to mind). it is possible to host more than 70 Linux/BSD environments on a single, self-diagnosing, multiply redundant, massively parallel host, that can dial your 4-hour-response support vendor automatically when it senses a component about to fail.
now, consider smaller firms. almost 60% of the US economy is an aggregate of SMBs. each of these is an organization whose entire IT budget is likely to be under one million USD, and that's on a good year, when they can afford an IT budget and don't decide to handle technology expenses on an ad-hoc basis (you'd be alarmed how often that happens). someone mentioned a savings of 500K british pounds. this figure often exceeds an SMB's entire IT budget, and in many cases, exceeds the entire organizational operating budget for the year.
other arguments include "thousands of corporate lemmings can't be wrong" (please examine any of the technology bubbles of the last 20 years to see how this reasoning fails), and "statistics indicate that it will be up forever" (MTBF statistics have a more tenuous relationship with reality than most people like to think).
the one application i can think of, that would be most compelling from a cost-benefit perspective, and involves minimal complexity, is to have two large, beefy, multi-socket and multi-core boxes running fail-over VMs for many smaller physical dedicated single-app VM servers. that way, one gets close-to-unvirtualized performance on one's dedicated hardware, but each dedicated box still has a VM host to fail over to, if needed. this setup may also offer benefits from a licensing perspective, depending on the software used.