Ugh, it ate my post.
You'll see two main differences with 64 bit code. The biggest reason systems have been moving to 64 bit is memory. A 32 bit application can only address 4GB of memory (although they are ways around this). If you find your photoshop usage is normally below 4GB the upgrade won't make too much of a difference for you. The second is that you can see a performance increase in certain types of applications. These aren't anything like a 2x speedup, maybe 20-30% at best. Photoshop can benefit and I'm sure Adobe has done some. If anything the 64 compiler can make some optimizations even if the developers haven't done anything specifically to support it yet. I'm sure there is a performance boost, but accessing more then 4GB of memory was most likely the driving factor. Something like a new complier could possibly give more warnings, but both companies are probably using the newest optimized compilers so it's not really an issue. There are other benefits too, like being to access larger drives, access more files etc, but these are things only enterprise companies have really run into problems with.
As for Carbon, Apple basically had two APIs that did basically the same thing, but not quite (part of the reason for all this trouble). It's confusing to try to explain to developers which API to use and why you can or can't do somethings in one or not the other. It's expensive to keep both up to date, bug free and playing with each other properly. Apple did actually port Carbon to 64 bit, but dropped it at the last minute, so it was probably a hard decision. For the sake of a better designed API and a clearer future for OS X programming they had to drop Carbon at some point and they choose to do it with the change to 64 bit. I don't know the details on this part, but there were many graphic APIs and much of quicktime that weren't available in Cocoa, so that's probably one reason companies like Adobe didn't try to move, or couldn't move earlier. Besides, your whole programmer base is experienced with Carbon. These weren't added into OS X until Leopard, so even including the developer copies it hasn't been available for a long time.
Although it's difficult now, it should really show itself to be a good decision down the road. Compared to Microsoft who has actually reimplemented old bugs into their new OSes to keep developers happy who have software that relied on exploiting them for years. I'd had to do that for some software I've written and it's a PITA whenever you need to touch that section of code again. In a couple cases it made planned improvements or optimizations impossible.
You'll still see benefits of more then 4GB of ram when running multiple applications so those extra slots on your Mac Pro aren't a waste.
EDIT: and I was beaten to it.
Although, I haven't exactly seen stability benefits in moving to 64-bit from a software point of view. Is it hardware specific or have I just been blind to it?
