The truth is that nobody here knows how long Snow Leopard will be supported. My *guess* would be a lot less time than chscag thinks -- machine attrition will take care of most SL users over time, and ML adoption is already massive:
(handy chart from January, and
accompanying article):
As you can see, the base is basically all on Snow Leopard and higher, which means they have access to the Mac App Store, which means they can buy the upgrade to ML or whatever comes after ML.
I would recommend to people who CAN upgrade to Mountain Lion to do so -- it's cheap, it's good, and you will significantly reduce any chance of problems when you upgrade to 10.9 or whatever they're going to call it.
If your hardware is quite old and/or just barely meets the required spec for ML, and you know you're going to be replacing it soon, you might opt to just stay where you are. I have a 2007 BlackBook that ran Lion just fine but if I were to sell it I'd put Snow Leopard on it for the best experience, given it's limited RAM capacity and lesser graphics card.
There's not really any path to Lion anymore, so that avenue is essentially closed and Lion numbers will fall significantly by year's end, you can rest assured. Apple could be making the update to Mountain Lion a LOT more "mandatory" than they have thus far, but they are apparently comfortable as long as the vast majority of the base are at least on Snow Leopard, which set the stage for everything that has come since.
