Ok, my quick review:
Photoshop - Very stable, most features disabled. You can adjust exposure, offset, gamma and use the channel mixer. Blur and sharpen work, layers do not. Clone tool works and thats about it. CS3 should allow more options (I will download the Beta soon).
PixelImageEditor (PIE) - Very unstable, supports layers and all of its other tools apparently. Need more time to evaluate.
Cinepaint - No luck with this.