Not entirely. See, here is what Adobe says is enhanced using the GPU:
GPU Feature List
OpenGL/GPU features in Adobe Photoshop CS4 include the following:
Smooth display at all zoom levels
Animated Zoom tool
Animated transitionsfor One Stop Zoom
Hand toss image
Birds-eye View
Rotate Canvas
Smooth display of non-square pixel images
Pixel grid
Move color matching to the GPU
Draw Brush tip editing feedback via the GPU
3D GPU features include the following:
3D Acceleration
3D Axis
3D Lights widget
Accelerated 3D interaction via Direct To Screen
Bridge GPU features include the following:
Preview panel
Full-screen preview
Slideshow
Review Mode
Of course what's available is dependant on the power of your card:
Hardware/Application Combinations and Available Functionality
A) Basic GL Drawing
Panning, Zooming, Rotating, Direct to Screen Non-Color Matched.
B) Basic GL Drawing with Color-Matched Direct to Screen 3D
Panning, Zooming, Rotating, Color-Matched Toned Direct to Screen 3D
C) Advanced GL Drawing
Panning, Zooming, Rotating, Accelerated Color Matching, Toning on Card
D) Advanced GL Drawing with Color-Matched Direct to Screen 3D
Panning, Zooming, Rotating, Accelerated Color Matching, Toning on Card
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MacTel with Shader Model 3.0 + OpenGL 2.0 card (may vary from 10.4.11 to 10.5.2 since Apple updated the drivers)
<= 128 Megs
Photoshop Standard supports A; very few GL windows due to very low texture memory.
Photoshop Extended supports B; very few GL windows due to very low texture memory.
<= 256 Megs
Photoshop Standard supports D, defaults to A
Photoshop Extended supports C
> 256 Megs
Photoshop Standard supports D
Photoshop Extended supports D
> 512 Megs: Uses more texture memory to better handle larger images.
So my assertion is that there are gains to be realized, yes indeed, but if you are working on simple stuff one/two layer banners and sigs, there is no killer functionality that cannot be gleaned from the old school photoshop methodology of doing everything via CPU.