Turns out CS4 is the Snow Leopard Problem Child
forums.adobe.com
A forum thread documenting common, repeatable bugs in Adobe Photoshop CS4 running in Snow Leopard. If you need to use Photoshop, DON'T UPGRADE YET!
forums.adobe.com
And for two, Apple have been accommodating Adobe's needs for years, with little to no reciprocity. The sole reason for the existence of Carbon is so Adobe and Microsoft wouldn't have to make significant changes to their products overnight. In return, Adobe dragged their feet with Carbonization, the switch to OS X, the switch to x86, and now the switch to 64-bit. In the last nearly 10 years Adobe had not put one iota effort into transitioning their core products to Cocoa. They had put the bare minimum of effort into optimizing their products for Mac OS X (in some cases, like the Flash plugin not even that).
Why should Apple go out of their way to ensure compatibility with Adobe's CS, when Adobe don't seem to care enough to test and fix things on their end?
(There is an apocryphal story about the reason for the rift between the two companies - back in the early days of Rhapsody Apple had a three way meeting with Adobe and Microsoft in which they discussed Apple's API plans and in which Apple tried to push the two biggest software vendors to port their products to Cocoa (née NextStep). Surprisingly, it supposedly was Adobe not Microsoft, who although not thrilled at the idea were ready to find a compromise, that put the kibosh on Apple's plans. They outright told Apple to go pound sand - they didn't intend to put effort and money in such a port, regardless of the fact that Adobe had a version of Illustrator for NextStep, meaning they had both the expertise and the code base to do it. Therefore Carbon.)
It's causing them problems now, but it's the basis for their successes.
So overall it was the right thing to do. (Despite, me, and every programmer who hears about it, cringing at the idea.)
For way, way more on the topic: http://www.joelonsoftware.com/articles/APIWar.html and for the gritty specifics of many of the hacks: http://blogs.msdn.com/oldnewthing/
Given that there's a finite range of Apple hardware that Snow Leopard is even compatible with, one would think that testing for repeatable bugs like this would have started months ago--and finished before Adobe signed off on CS4 for use with Snow Leopard.
One user reported that on a clean install with CS4, everything went smoothly. That, to me, says some people have stuff running that is causing problems.
I had issues with system stability after the upgrade. Lot's of beachballing. Turned out to be a kernel extension that I was using in Leopard. Hardly surprising that a rogue kernel extension can make your system unstable. After removing it, SL has been rock solid.
I don't care what people here will say, gimp doesn't cut it in some major area's including things like photography and camera raw.
Gimp is darn good but missing 30% of the sort of features that make me hand over a wudge of cash while shouting "It must be mine!"
It's quite possible that someone has a minority experience, where it doesn't work. First steps are to check the integrity of the applications (it's legit CS4 direct from Adobe?), then any customizations to the system that may cause it to act differently than others. It's to everyone's advantages to document all meaningful Snow Leopard differences.
Also, following someone's Twitter stream for information is fail when it's a ton of @ messages.