64-bit Photoshop not coming yet

I've gotten a number of questions on the beta forums as to why Photoshop CS3 won't have a 64-bit version. It's definitely a when question, not an if, and there are a lot of factors involved. I though I might collect some of the information together here.

First, let's check all the 64-bit hype at the door. Being a 64-bit program means, most simply, that pointers in an application are 64 bits wide, not 32 bits. A 32 bit pointer means that an application can address 2 ^ 32 bytes of memory, or 4GB, at the most. The operating system an application runs on slices that application address space up, so that the application can actually only allocate a part of that address space for itself...