I played around with the soft proofing feature in LR4b last night. Even after watching the Julianne Kost video a couple of times and knowing the CS4 soft proofing features, I'm still a bit confused. I didn't go a far as to start printing screens to see what things looked like. Here are my concerns.
I figured out how to get the screen to emulate my paper by clicking on the 'Soft Proofing' box in the screen and the 'Simuate Paper & Ink' in the panel. This gets me to two screens that compare my adjusted image to the printer output image. I can then adjust the printer output image to match the adjusted image, but after closing all this and going back to the image, it is the adjusted output image. It would seem that I should be adjusting the output image only and keeping the adjusted image as it was.
I am aware of the save feature that pops up. I'm assuming that what you are doing is creating a new image in metadata that has all the printer adjustments. But the save also impacts what is in the panels (original, adjusted, proof, more?). It gets confusing.
It's also not clear to me what is actually going to print. I'm assuming it image in the 'Proof Preview' panel, which has all the adjustments for the paper or other media that is going to go to output.
My take on all of this is that if the proof is reasonably accurate to the output media, this is fabulous feature. It is tricky, though, and I think either a set of very good instructions, or possibly some changes to the controls and labels would be helpful.