A follow-up to this and another thread where I was commenting on clipping the blue channel when exporting an image from Lightroom to Photoshop.
I now am using Lightroom 2.5 and PS CS4 and still find that an image that gives me a good histogram in LR shows significant clipping on the shadow end of the blue channel. And this is with LR set to Export to the Adobe RGB profile.
I discovered today that by simply applying a Vibrance adjustment of even +1 or -1 in PS, the blue channel clipping suddenly disappears. Not sure what's happening here, and there's no apparent visible change in the image on my monitor.
The B channel clipped in the exported image means Adobe RGB couldn't keep all the colour gamut when converting from LR's colour space. That means you already lost that information in the B channel.
If now in PS, by desaturating the B clipping disappears, is just an illusion in terms of quality. The information was already lost. The variations in that channel that were present in LR, are not present in PS anymore. Desaturating increases the B channel in those pixels to some positive value, that's why it looks non-clipped. But the hurt was already done so you are not fixing the problem here.
If you can't see differences, is because they are actually very slight and impossible to notice in a standard monitor. Maybe this last sentence is the one to take into account from a practical point of view.