Last fall I forgot to set the time on my camera back an hour. I used jhead to change the time in the EXIF data:
jhead -ta-1 IMG_1177.JPG
but Picasa didn't notice. OS is Windows XP.
If I right-click the picture in Picasa and choose Properties, in the top section of the dialog it shows "Date: 3/6/2010 11:22:54 AM" but in the Property|Value section it shows "Camera Date | 2010:03:06 10:22:54".
I tried using "Refresh thumbnails" but that didn't help. Picasa notices other changes, like if I edit the photo in the Gimp or change the filename, but for some reason it doesn't notice this change.