The image explains it all.
CMYK works fine, but RGB doesn't. The preview (in the channels pane) looks fine though.
Pixelmator had the same problem, but I fixed it by turning off color management in Pixelmator.
My question is, how can I fix this problem in a legal copy of Photoshop CS5 Extended?
My Monitor is a dell e193fp, and am running Mac OS X 10.7.2.
My video card is an ATI Radeon HD 4600, and I have a Mid-08 Mac Pro.
It could be your display profile, or a problem with the video card drivers.
I'd bet on the profile.
Also: RGB is not a colorspace. RGB is a color model, or color mode. A model or mode describes the number of channels, not the meaning of the channels.
sRGB and Adobe RGB are colorspaces. A colorspace describes the meaning of the color channels (a specific red, a specific green, a specific blue, a specific white point, a specific tone curve) for a given mode or model.
For a color mode or model, there can be many spaces.