That is true, that apple has an incentive to make the new OS not work on old hardware. I have a 2006 Mac Pro (first model with Intel chips) that ran me ~$2800 back in August 2006. It's still chugging along great, doing my video transcoding work when I ask it to do so -- albeit not as quickly as a brand new machine, of course.
I don't use it much anymore, but it still works fine, as does my Dell monitor I bought at the same time.
The Mac Pro is not capable of running Mountain Lion. That's a bit of a bummer, as I like having all of my machines on the same OS. However, I don't use the system a lot, and Lion will work fine on it. It's 6 years old. I'm sure in another 2 years I'll stop using it entirely, but 8 years is a pretty good run.