Probably the same thing as happens with any other subscription or
rental service - the service stops.
But the service is only available in Brazil so far, with plans to
introduce it to India, Mexico, Russia and China over the next several
months.
There is more detailed information available at
http://www.microsoft.com/whdc/flexgo/payasyougo.mspx
There are apparently two different plans being introduced, according
to that site. One is essentially a monthly installment purchase of a
PC plus Windows through an Internet Service Provider with 20 percent
down and the balance paid over 24 months. After 24 months the computer
is paid for and the ISP's monthly fees would be reduced accordingly.
The second is a prepaid card system based on actual usage. That one
is based on a one-third down payment with the balance of the price
paid over 800 hours of usage. Prepaid usage cards would be purchased
and inserted into the computer while it is being used. When the hours
on the card are used up a new card would be have to be purchased with
more hours on it. After 800 total hours have been used the cards
would no longer be required.
There is, by the way, at least one other software rental system
available from Microsoft for certain users. That is the Microsoft
Action Pack