One more from me!
I sort of found a way to do this, posted about here:http://thedambook.com/smf/index.php?topic=4878.msg27420#msg27420
Now I am trying to get my ACR edits, made in Bridge to the DNG, to show up in Expression. I started with a JPEG, converted it to DNG through Camera Raw with no changes.
I then made obvious edits in ACR to the DNG in Bridge/Camera Raw, clicked Done. The thumbnail in Bridge updated.
The folder was not yet in my EM2 catalog, so I imported it. The thumbnail for the DNG did not reflect my edits. I tried Rebuild Item; no change.
If I make the PIE edits in Camera Raw PRIOR to saving the image as DNG, they do show in the thumbnail when importing to EM2. However, if I make a further PIE edit in Camera Raw, and click Done to dismiss ACR, the thumbnail doesn't change in EM2. I tried Rebuild Item; no change.
I then removed the folder from the catalog and re-imported. My original PIE edits showed up, but not the second set. Again, no change with Rebuild Item.
If I choose Save Image within ACR (instead of "Done") I get a second DNG -- it doesn't replace the first one -- and this second DNG does reflect my most recent PIE edited in the EM2 thumbnail.
So, all of this is basically evidence in favor of Peter's recommendation in the DAM book that you make your edits before cataloguing in EM2; once they're in the catalogue, that is the "permanent" home and if you make PIE edits later it should be treated as a derivative file workflow.
Again... Picasa sucks for a lot of reasons... but for PIE editing on the fly, albeit with a limited toolset, it was super smooth... grass is always greener, I guess.