Check the EXIF data on the photo if you edit the tag with the software. I'm using the EXIF Viewer 1.55 addon for firefox to see them. I added a tag to a photo in the Picasa software and it put the tag in the IPTC Keywords.
When I view the file properties in Windows it doesn't show. However if you edit the keywords in the advanced properties of the file in Windows it will add a section to EXIF IFD0 called Microsoft.XP.Keywords with what you added that way.
I just did this on a couple of jpg's on my system, should work the same way for you.
Looking at the note you're referring to, its the face tag data which makes sense. The face tagging also tags the location of the person's face in picture. I'm not sure there's a place for that kind of metadata right now in the file.
Here we go, just found this response in the help forum for Picasa:
Why are picasa comments not saved in exif or IPTC header fields in the image file? - Picasa Help
"Best answer - ajr225 (Asker) Go to this answer
Picasa "captions" and "tags" are saved in the IPTC data in images that support IPTC data. JPEGs for example. "
Are you tagging RAW files or jpg? Picasa doesn't support putting metadata in all types of RAW files.
Edit: Looks like Picasa doesn't support the newer standard for the data though. People in that thread are talking about using command line tools or possibly something like
http://www.irfanview.com/ to edit the newer version of the keywords/tagging. Basically IPTC-Core vs the older IPTC-IIM