ALL they've done is rewrite the code to give a solid platform for their hardware platform going forward. That's it. It really doesn't affect your existing setup. About the only thing I can think of is latest software releases might not work but when you're talking a machine over three years old then compatibility of the latest software is always a little bit questionable. And considering that along with this update Apple have also issued security fixes for Tiger I'd suggest you have at least another year of support coming to you, if not more which would take you to your five year self-imposed limit.
Let me repeat what you said in simpler terms...
Major Bug Fixes for INTEL users who paid $129 for Leopard for just $29.
End of Major Bug Fixes for PowerPC users who paid $129 for Leopard at the same time.
But continued security fixes for Leopard just to prevent lawsuits basically.
And lets not get into the whole "new features that won't work on PowerPC" thing.
That's already been thoroughly debunked and some of Snow Leopards new features won't even work on INTEL graphics machines.
A previous poster summed it up best... There's no reason to upgrade your computer every 2-3 years anymore. We've topped out at 3Ghz basically. A new computer is not always significantly faster than the previous one. My Powermac G5s that are 3 years old are probably still faster than most of the Mac Minis ever made and will continue to be quite sufficient to do not just simple tasks, but even some complex ones.
Apple is abandoning at least one quarter to one third of their user base with Snow Leopard.
By the way, I didn't see any of the PowerPC issues I've had in the past few months addressed in 10.5.8. I file Safari 4.0.2 crash reports every couple days, so I doubt 10.5.8 is going to help, but we'll see. Firefox crashes too, but much more rarely and I use both at the same time often, so its a fair comparison.