The Gorse Fox has tried for the second or third time to move his photo library to the MAC. The movement of the photos (all 50GB) is moderately straightforward. The issue lies with the recovery of the albums, edits, and face tags that had been assigned on the PC.
It would appear that whilst the Picasa Backup and Restore will (with the help of a crowbar) restore the whole environment to the MAC, the face recognition information regarding the geometry of the face tags (i.e. where the identifying rectangle is on the photo) seems to become randomly screwed up. GF suspects it is a feature of whether the tag was assigned before or after the photo was straightened or cropped. It is, however, causing some strange results and is about to trigger a significant bit of work!
Now, in his innocence, he thinks this is a design flaw. All tags, edit decision lists, or identifying information that involve or reference the image geometry should use the original image as the origin and inherit relative references in any cascading edits.