In the past I felt there was a reasonable consistency between what my camera showed as overexposed by blinkies and what I saw in Lightroom. Now with PV 2012 i LR4 this consistency has totally gone! I see in a number of pictures that a full stop higher exposure will show sometimes less blinkies than the one stop lpwer exposure on the LCD screen on the camera. My camera is a Canon 1Ds mkIII and I assume that a blinky is generated if at least one channel is blown out. I also assume that an area shown as overexposed in Lightroom (by pushing the J-key) also is one where at least one channel is blown out. In other words all other areas in the photo not shown as overexposed in LR do not have any channel blown out. So now the question: How can an area that is shown is partially blown out in LR3 or PV2010 be shown as not blown out at all in PV2012?
I agree with all of what Bart posted, but would like to add a few points. According to Eric Chan, PV2012 automatically performs highlight recovery and Jeff Schewe appears to be ambiguous on this topic. As usual, tests with your own camera are in order. As an example, here are a few tests I did with the Nikon D3 by exposing a Stouffer wedge and comparing the actual raw histogram as shown by Rawnalize and Rawdigger with the camera histogram and blinking highlights as well as the ACR histogram and highlight indicator. I used the Nikon standard picture control (normal contrast).
This image demonstrates that the Nikon histogram is slightly conservative, but close enough for practical work.
The blinking highlights are a bit more conservative.
The ACR histograms in PV2010 are complicated by the +0.5 EV exposure offset used by ACR for this camera. To compensate for this offset, it is necessary to decrease the ACR exposure by 0.5 EV.
Here is PV2010 with no exposure compensation for image 05 which is slightly below clipping as shown by the Rawnalize histogram. It appears blown in ACR.
Using the -0.5 EV compensation, the image still appears slightly overexposed.
PV2012 shows a more accurate histogram for the properly exposed image. The baseline offset seems not to be in effect.
Image 03 has clipped green channels. The blue channel is at clipping and the red channel is just short of clipping (this is with a 5000K light box).
The PV2012 histogram exhibits minimal clipping, indicating the effect of highlight recovery.
I don't know what baseline offset PV2010 uses for your camera (this can be determined by converting to DNG and using an exif reader), but for my own camera, PV2012 shows improvement.