What the Tesla Resale Value Guarantee means is that, when a private or business customer gets a Model S as part of a 36-month lease, the company guarantees "that the resale value of Model between months 36 and 37 will be among the highest of any premium saloon made in volume" (Saloon is British carspeak for sedan). The deal is not applicable retroactively to Model S owners who have already taken delivery of their car.
There are some changes for the global program, of course, compared to the original. In the US, Tesla uses ALG resale value numbers to determine the value, while in Europe it will use the "average residual values as forecasted by CAP and EuroTax, independent third parties" (in the UK, at least). There's more information here and then the fine print details here.
In 2013, Bloomberg Industries estimated that Tesla's guaranteed buyback program could generate $368 million in revenue for Tesla due to the resulting used vehicle sales in 2016. Late last year, we heard that the automaker is indeed working on a certified used car program.