>> Can programs such as Photoshop CSx or Gimp be easily recompiled to run under CUDA or will special versions have to be written?.
Not really. But computationally intensive algorithms could be cast off to the GPU while the CPU is handling other aspects of application processing.
Although Photoshop would not run from the GPU, things like filter effects, video encoding, and image scaling subsystems and plugins could leverage the GPU’s parallel processing technology resulting in total performance improvement. In other words, certain blocks of application functionality could execute on the GPU while the CPU services the bulk of the application’s general I/O and GUI requirements.