What is the typical lifetime of a PS version with respect to changing Mac operating systems? For instance, if I purchased CS5 today, based on recent history would it likely be compatible with 10.7 when that is released? 10.8?
How often do you need to upgrade PS in order to remain compatible with Mac OS changes?
I can still run Photoshop CS2 on my brand new MacPro with OS10.6.6. That's 6 years. I expect CS would also run if I were to install it.
While you may experience some minor issues after an OS upgrade, for the most part Adobe apps are pretty forward compatible.
The truth is there is no absolute answer to your question. No one knows what 2 or 3 years in the future will hold. Not even Apple and Adobe. The best you can do is get the tools you need today. No one ever forces you to upgrade and OS or application. You should always then research before upgrading anything.
On today's systems it's pretty easy to do dual boot for a new OS release and "test. That's generally what I do with new OS releases - install on a spare drive and test how well the OS and applications work. This always allows me to boot back to a configuration I know is solid and working.
Also be aware, live versions matter. For instance if Apple were to release OS10.7 tomorrow and there were issues with Photoshop CS5. Adobe would work on or release a CS5 update. However, they won't ever release another CS3 update.