Whether you become 'un-upgradeable' is more determined by the hardware/Mac you have, rather than the OS you are currently running.
The early core2 (not duo's) can't officially upgrade to the latest OS. I'm running one of the last core2 duo's and expect that some future OS won't run on that. Could even be whatever OS follows Mountain Lion.
It really is hardware dependent on whether we get left behind or not.
At the moment you can upgrade from 10.6.8 to 10.8.3, limited of course by the hardware you have and as explained by Paul_31.
The caveat here is that when Apple launch a new OS it might not be possible to jump from 10.6.8 to this new OS, because (and as in the case with OS X 10.5 Lion) the older OS is usually removed from the App Store a little while after the new version is lauched.
Ok, thanx to Paul_31 & Zlig ! Hardware Overview:
Model Identifier: iMac10,1
Processor Name: Intel Core 2 Duo
Processor Speed: 3.06 GHz
Number Of Processors: 1
Total Number Of Cores: 2
What Zlig says confirms what I'd heard, that once Apple launch a new OS ( 'Tabby' ?) my Mac will become un-upgradeable, so that'd be a reason to fork out the $20 for Mountain Lion, tho' some of my 3rd-party/browser add-ons/apps won't work w/ that OS, but at least I'd stay 'current' ?