I haven't seen this mentioned before (apart from a little discussion re sharpening in the Lightroom thread), so I thought I'd post this heads-up.
If, like me, you've got used to using ACR to perform capture sharpening and you open the raw file as a smart object in PS, then be careful when going back to old psd files. The new 2010 demosaicing algorithm applies far more aggressive sharpening than the old 2003 process, and ACR6 uses it by default.
Try the following: (See my notes at the end, this will be most noticeable on noisy images from APS-C and below sensors.)
1) Open an old psd file that is based on a raw which has had sharpening applied in ACR and been saved as a smart object.
2) Click on the raw file's layer icon to re-open it in ACR.
3) Do nothing.
4) Click OK.
Once PS has updated the smart object, your image will now be grossly over-sharpened, since the sharpening controls will remain unaltered from the values suitable to the old process. You can re-open the smart object in ACR and set the process back to 2003 in the Camera Calibration tab. This brings the sharpening process back to where it was before, but it doesn't seem to be entirely faithful - I've found a couple of images where getting back to the original state required dialling-back the detail slider and this is most noticeable on images that didn't have any luminance noise reduction applied.
Most annoyingly, if you've set it back to '2003' and you then reopen the smart object in ACR again, the process will revert back to 2010, so if you want to stick with the old algorithm and settings you need to set it to 2003 every time you reopen the smart object. If you didn't use ACR to perform capture sharpening, then the changes are much more subtle - the new algorithm will produce some small changes that are generally only visible at a pixel level.
Note that this only happens if PS re-renders the smart object (when you get the 'Preparing Smart Object' progress bar). If you simply edit other layers then there's no change in the base image coming from the raw file. But as someone who thought he was being really smart by developing a workflow based around smart objects and smart filters, this development is a little galling.
I sincerely hope that ACR6.1 will detect the process used previously in a smart object and set it accordingly. I also hope that 6.1 fixes the '2003' process option so that it really does revert back to the way it was sharpened before without any need to twiddle the detail slider.
Aside from the shock that this induced, I must say that the new '2010 process' sharpening algorithm seems very nice, once you've got used to it. For APS-C sensors you'll probably want to dial-in a bit of luminance noise reduction even on shots that you previously considered clean, but shots from a large sensor sharpen very nicely, and the new noise reduction process is a marked improvement. In general I've found I need to set the amount lower than I'm used to, and the detail slider is particularly sensitive to noise.
[Edit]BTW, I forgot to mention: this only applies to raw files edited through the smart object function. Raw files that have previously been developed in ACR and had the settings stored in an xmp file will automatically revert to the original 2003 process if you open the raw file directly in PS.