It's my understanding that they've been there for years. IBM will install a mainframe with N cores even if a company only wants to pay for N/2 cores initially. Then, the company can ask IBM to turn on the switch (or just turn it on themselves and be honest?) and increase its capacity. No idea if they can throttle back, say after Christmas, though. Of course, it's not a perfect parallel as IBM parks the whole thing in the customer's own datacenter. I think there may also be some pay-for-mips options as well.
IBM still charges big banks CPU time on the bank's own hardware.
Big bank backend apps are written to sleep as much as possible, and then do everything all at once in order to save on CPU costs.